diff --git a/mlir/lib/Conversion/SCFToStandard/SCFToStandard.cpp b/mlir/lib/Conversion/SCFToStandard/SCFToStandard.cpp
--- a/mlir/lib/Conversion/SCFToStandard/SCFToStandard.cpp
+++ b/mlir/lib/Conversion/SCFToStandard/SCFToStandard.cpp
@@ -407,8 +407,11 @@
 void SCFToStandardPass::runOnOperation() {
   OwningRewritePatternList patterns;
   populateLoopToStdConversionPatterns(patterns, &getContext());
+  // Configure conversion to lower out scf.for, scf.if and scf.parallel.
+  // Anything else is fine.
   ConversionTarget target(getContext());
-  target.addLegalDialect<StandardOpsDialect>();
+  target.addIllegalOp<scf::ForOp, scf::IfOp, scf::ParallelOp>();
+  target.markUnknownOpDynamicallyLegal([](Operation *) { return true; });
   if (failed(applyPartialConversion(getOperation(), target, patterns)))
     signalPassFailure();
 }
diff --git a/mlir/test/Conversion/SCFToStandard/convert-to-cfg.mlir b/mlir/test/Conversion/SCFToStandard/convert-to-cfg.mlir
--- a/mlir/test/Conversion/SCFToStandard/convert-to-cfg.mlir
+++ b/mlir/test/Conversion/SCFToStandard/convert-to-cfg.mlir
@@ -1,4 +1,4 @@
-// RUN: mlir-opt -convert-scf-to-std %s | FileCheck %s
+// RUN: mlir-opt -allow-unregistered-dialect -convert-scf-to-std %s | FileCheck %s
 
 // CHECK-LABEL: func @simple_std_for_loop(%{{.*}}: index, %{{.*}}: index, %{{.*}}: index) {
 //  CHECK-NEXT:  br ^bb1(%{{.*}} : index)
@@ -398,3 +398,17 @@
   }
   return %0#0, %0#1 : f32, i64
 }
+
+// Check that the conversion is not overly conservative wrt unknown ops, i.e.
+// that the presence of unknown ops does not prevent the conversion from being
+// applied.
+// CHECK-LABEL: @unknown_op_inside_loop
+func @unknown_op_inside_loop(%arg0: index, %arg1: index, %arg2: index) {
+  // CHECK-NOT: scf.for
+  scf.for %i = %arg0 to %arg1 step %arg2 {
+    // CHECK: unknown.op
+    "unknown.op"() : () -> ()
+    scf.yield
+  }
+  return
+}