diff --git a/gcc/fortran/ChangeLog b/gcc/fortran/ChangeLog
index 950a0ee4564..2fccd4862f8 100644
--- a/gcc/fortran/ChangeLog
+++ b/gcc/fortran/ChangeLog
@@ -1,3 +1,9 @@
+2008-01-06  Tobias Burnus  <burnus@net-b.de>
+
+	PR fortran/34654
+	* io.c (check_io_constraints): Disallow unformatted I/O for
+	internal units.
+
 2008-01-06  Tobias Burnus  <burnus@net-b.de>
 
 	PR fortran/34660
diff --git a/gcc/fortran/io.c b/gcc/fortran/io.c
index bb4295b866a..0bff511db06 100644
--- a/gcc/fortran/io.c
+++ b/gcc/fortran/io.c
@@ -2669,6 +2669,11 @@ if (condition) \
 		     "REC tag at %L is incompatible with internal file",
 		     &dt->rec->where);
 
+      io_constraint (dt->format_expr == NULL && dt->format_label == NULL
+		     && dt->namelist == NULL,
+		     "Unformatted I/O not allowed with internal unit at %L",
+		     &dt->io_unit->where);
+
       if (dt->namelist != NULL)
 	{
 	  if (gfc_notify_std (GFC_STD_F2003, "Fortran 2003: Internal file "
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 5036598e140..b456bba426a 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2008-01-06  Tobias Burnus  <burnus@net-b.de>
+
+	PR fortran/34654
+	* gfortran.dg/internal_io_unf.f90: New.
+
 2008-01-06  Tobias Burnus  <burnus@net-b.de>
 
 	PR fortran/34660
diff --git a/gcc/testsuite/gfortran.dg/internal_io_unf.f90 b/gcc/testsuite/gfortran.dg/internal_io_unf.f90
new file mode 100644
index 00000000000..227b0267cb8
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/internal_io_unf.f90
@@ -0,0 +1,11 @@
+! { dg-do compile }
+!
+! PR fortran/34654
+!
+! Disallow unformatted write to internal unit.
+! Test case was contributed by Joost VandeVondele.
+!
+implicit none
+CHARACTER :: a(3)
+WRITE(a) 0 ! { dg-error "Unformatted I/O not allowed with internal unit" }
+END