diff --git a/CHANGELOG.md b/CHANGELOG.md
index d370d5e..e9bb60e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+### Fixed
+
+- `validateStateDefinitionParallelSubactions` and `validateStateUsageParallelSubactions` report only transitions in a parallel state, not successions: SysML 8.3.18.5 and 8.3.18.6 forbid incoming and outgoing transitions of the substates, and 7.18.2 and 7.18.3 say the same. The Pilot Implementation also rejects a succession there; the opensysml fixtures `s33-parallel-state-usage-with-succession` and `p19-parallel-state-with-transition`, which hold a succession and no transition, move to unknown in both baselines.
+
 ## v0.11.3
 
 ### Added
diff --git a/crates/sysmlv2-model/src/check.rs b/crates/sysmlv2-model/src/check.rs
index da39904..59ce3bc 100644
--- a/crates/sysmlv2-model/src/check.rs
+++ b/crates/sysmlv2-model/src/check.rs
@@ -1168,6 +1168,29 @@ mod tests {
         assert_eq!(shown(&on_threads), shown(&in_line));
     }
 
+    /// SysML 8.3.18.5 validateStateDefinitionParallelSubactions and
+    /// 8.3.18.6 validateStateUsageParallelSubactions forbid transitions of
+    /// the substates of a parallel state, not successions (7.18.2).
+    #[test]
+    fn a_parallel_state_may_own_successions() {
+        let mut model = Model::new();
+        model.add_source(
+            "user.sysml",
+            "package P {
+                state def S parallel { state a; state b; first a then b; }
+                state t parallel { state a; state b; first a then b; }
+                state def T parallel { state a; state b; transition first a then b; }
+                state u parallel { state a; state b; transition first a then b; }
+            }",
+        );
+        let found: Vec<String> = findings(&model)
+            .into_iter()
+            .map(|(_, m)| m)
+            .filter(|m| m.contains("ParallelSubactions"))
+            .collect();
+        assert_eq!(found.len(), 2, "{found:?}");
+    }
+
     #[test]
     fn semantic_checks_walk_tables_without_copying_them() {
         // Fresh build: library rows and user rows share one storage.
diff --git a/crates/sysmlv2-model/src/check/structural.rs b/crates/sysmlv2-model/src/check/structural.rs
index 3750e77..9cc1a61 100644
--- a/crates/sysmlv2-model/src/check/structural.rs
+++ b/crates/sysmlv2-model/src/check/structural.rs
@@ -582,9 +582,9 @@ pub(super) fn validate(r: &ResolvedModel, model: &Model, g: &Facts) -> Vec<(usiz
                 }
             }
             if flag(b, e, "isParallel")
-                && g.members[e].iter().any(|&m| {
-                    is_a!(b.elements[m], "TransitionUsage") || is_a!(b.elements[m], "Succession")
-                })
+                && g.members[e]
+                    .iter()
+                    .any(|&m| is_a!(b.elements[m], "TransitionUsage"))
             {
                 report(
                     e,
@@ -594,7 +594,7 @@ pub(super) fn validate(r: &ResolvedModel, model: &Model, g: &Facts) -> Vec<(usiz
                     } else {
                         "validateStateUsageParallelSubactions"
                     },
-                    "A parallel state cannot own successions or transitions",
+                    "A parallel state cannot own transitions",
                 );
             }
         }
diff --git a/crates/sysmlv2-parser/tests/fixtures/opensysml/baseline-library.json b/crates/sysmlv2-parser/tests/fixtures/opensysml/baseline-library.json
index b0e256d..b9ae2d2 100644
--- a/crates/sysmlv2-parser/tests/fixtures/opensysml/baseline-library.json
+++ b/crates/sysmlv2-parser/tests/fixtures/opensysml/baseline-library.json
@@ -2702,16 +2702,8 @@
     "status": "diagnostic baseline"
   },
   "negative/semantic/s33-parallel-state-usage-with-succession.sysml": {
-    "diagnostics": [
-      {
-        "end": 196,
-        "message": "A parallel state cannot own successions or transitions [validateStateUsageParallelSubactions]",
-        "severity": "Error",
-        "stage": "semantics",
-        "start": 195
-      }
-    ],
-    "status": "diagnostic baseline"
+    "diagnostics": [],
+    "status": "unknown"
   },
   "negative/semantic/s34-state-usage-two-entry-actions.sysml": {
     "diagnostics": [
@@ -3422,16 +3414,8 @@
     "status": "accepted (adjudicated)"
   },
   "negative/xpect/p19-parallel-state-with-transition.sysml": {
-    "diagnostics": [
-      {
-        "end": 215,
-        "message": "A parallel state cannot own successions or transitions [validateStateDefinitionParallelSubactions]",
-        "severity": "Error",
-        "stage": "semantics",
-        "start": 214
-      }
-    ],
-    "status": "diagnostic baseline"
+    "diagnostics": [],
+    "status": "unknown"
   },
   "negative/xpect/p20-connection-with-one-end.sysml": {
     "diagnostics": [
@@ -5416,7 +5400,7 @@
     "diagnostics": [
       {
         "end": 151,
-        "message": "A parallel state cannot own successions or transitions [validateStateDefinitionParallelSubactions]",
+        "message": "A parallel state cannot own transitions [validateStateDefinitionParallelSubactions]",
         "severity": "Error",
         "stage": "semantics",
         "start": 150
@@ -5452,7 +5436,7 @@
     "diagnostics": [
       {
         "end": 156,
-        "message": "A parallel state cannot own successions or transitions [validateStateUsageParallelSubactions]",
+        "message": "A parallel state cannot own transitions [validateStateUsageParallelSubactions]",
         "severity": "Error",
         "stage": "semantics",
         "start": 155
diff --git a/crates/sysmlv2-parser/tests/fixtures/opensysml/baseline.json b/crates/sysmlv2-parser/tests/fixtures/opensysml/baseline.json
index e5e723b..b87ca3f 100644
--- a/crates/sysmlv2-parser/tests/fixtures/opensysml/baseline.json
+++ b/crates/sysmlv2-parser/tests/fixtures/opensysml/baseline.json
@@ -2744,16 +2744,8 @@
     "status": "diagnostic baseline"
   },
   "negative/semantic/s33-parallel-state-usage-with-succession.sysml": {
-    "diagnostics": [
-      {
-        "end": 196,
-        "message": "A parallel state cannot own successions or transitions [validateStateUsageParallelSubactions]",
-        "severity": "Error",
-        "stage": "semantics",
-        "start": 195
-      }
-    ],
-    "status": "diagnostic baseline"
+    "diagnostics": [],
+    "status": "unknown without library"
   },
   "negative/semantic/s34-state-usage-two-entry-actions.sysml": {
     "diagnostics": [
@@ -3569,16 +3561,8 @@
     "status": "accepted (adjudicated)"
   },
   "negative/xpect/p19-parallel-state-with-transition.sysml": {
-    "diagnostics": [
-      {
-        "end": 215,
-        "message": "A parallel state cannot own successions or transitions [validateStateDefinitionParallelSubactions]",
-        "severity": "Error",
-        "stage": "semantics",
-        "start": 214
-      }
-    ],
-    "status": "diagnostic baseline"
+    "diagnostics": [],
+    "status": "unknown without library"
   },
   "negative/xpect/p20-connection-with-one-end.sysml": {
     "diagnostics": [
@@ -5878,7 +5862,7 @@
     "diagnostics": [
       {
         "end": 151,
-        "message": "A parallel state cannot own successions or transitions [validateStateDefinitionParallelSubactions]",
+        "message": "A parallel state cannot own transitions [validateStateDefinitionParallelSubactions]",
         "severity": "Error",
         "stage": "semantics",
         "start": 150
@@ -5914,7 +5898,7 @@
     "diagnostics": [
       {
         "end": 156,
-        "message": "A parallel state cannot own successions or transitions [validateStateUsageParallelSubactions]",
+        "message": "A parallel state cannot own transitions [validateStateUsageParallelSubactions]",
         "severity": "Error",
         "stage": "semantics",
         "start": 155