diff --git a/lightstep-haskell.cabal b/lightstep-haskell.cabal
--- a/lightstep-haskell.cabal
+++ b/lightstep-haskell.cabal
@@ -1,7 +1,7 @@
 cabal-version: 2.4
 
 name:           lightstep-haskell
-version:        0.4.3
+version:        0.4.4
 synopsis:       LightStep OpenTracing client library
 description:    LightStep OpenTracing client library. Uses GRPC transport via proto-lens.
 category:       Tools
diff --git a/src/LightStep/Propagation.hs b/src/LightStep/Propagation.hs
--- a/src/LightStep/Propagation.hs
+++ b/src/LightStep/Propagation.hs
@@ -48,7 +48,8 @@
 b3headersForSpanContext :: SpanContext -> [(BS.ByteString, BS.ByteString)]
 b3headersForSpanContext ctx =
   [ ("x-b3-traceid", encode_u64 $ ctx ^. traceId),
-    ("x-b3-spanid", encode_u64 $ ctx ^. spanId)
+    ("x-b3-spanid", encode_u64 $ ctx ^. spanId),
+    ("x-b3-sampled", "true")
   ]
 
 encode_u64 :: Word64 -> BS.ByteString