diff --git a/src/ltlparse/ltlparse.yy b/src/ltlparse/ltlparse.yy
index cfd20f356..6d55e5111 100644
--- a/src/ltlparse/ltlparse.yy
+++ b/src/ltlparse/ltlparse.yy
@@ -234,6 +234,8 @@ booleanatom: ATOMIC_PROP
 	      { $$ = constant::false_instance(); }
 
 rationalexp: booleanatom
+            | OP_NOT booleanatom
+	      { $$ = unop::instance(unop::Not, $2); }
             | bracedrationalexp
 	    | CONST_EMPTYWORD
 	      { $$ = constant::empty_word_instance(); }
diff --git a/src/ltlvisit/consterm.cc b/src/ltlvisit/consterm.cc
index 864442c6a..87f5afb7f 100644
--- a/src/ltlvisit/consterm.cc
+++ b/src/ltlvisit/consterm.cc
@@ -76,6 +76,11 @@ namespace spot
 	  switch (uo->op())
 	    {
 	    case unop::Not:
+	      result_ = false;
+	      break;
+	    case unop::Star:
+	      result_ = true;
+	      break;
 	    case unop::X:
 	    case unop::F:
 	    case unop::G:
@@ -84,9 +89,6 @@ namespace spot
 	    case unop::NegClosure:
 	      assert(!"unsupported operator");
 	      break;
-	    case unop::Star:
-	      result_ = true;
-	      break;
 	    }
 	}
 
diff --git a/src/tgbaalgos/ltl2tgba_fm.cc b/src/tgbaalgos/ltl2tgba_fm.cc
index 54d8c808c..b4dfec499 100644
--- a/src/tgbaalgos/ltl2tgba_fm.cc
+++ b/src/tgbaalgos/ltl2tgba_fm.cc
@@ -364,14 +364,22 @@ namespace spot
 	  {
 	  case unop::F:
 	  case unop::G:
-	  case unop::Not:
 	  case unop::X:
 	  case unop::Finish:
 	  case unop::Closure:
 	  case unop::NegClosure:
-	    break;
 	    assert(!"not a rational operator");
 	    return;
+	  case unop::Not:
+	    {
+	      // Not can only appear in front of constants or atomic
+	      // propositions.
+	      const formula* f = node->child();
+	      assert(dynamic_cast<const atomic_prop*>(f)
+		     || dynamic_cast<const constant*>(f));
+	      res_ = !recurse(f) & next_to_concat();
+	      return;
+	    }
 	  case unop::Star:
 	    {
 	      formula* f;
diff --git a/src/tgbatest/ltl2tgba.test b/src/tgbatest/ltl2tgba.test
index d073675dd..bb77e43af 100755
--- a/src/tgbatest/ltl2tgba.test
+++ b/src/tgbatest/ltl2tgba.test
@@ -78,7 +78,7 @@ check_psl 'G{(a;b)*}'
 check_psl '{a*}[]->{b*}'
 check_psl '{a*&b}'
 check_psl '{a*&b*}'
-check_psl '{((c;b*) & d);e}'
+check_psl '{((!c;b*) & d);e}'
 check_psl '{(a* & (c;b*) & d);e}'
 # Example from "Beyond Hardware Verification" by Glazberg, Moulin, Orni,
 # Ruah, Zarpas (2007).