diff --git a/Changelog.md b/Changelog.md
index d05ad4de3..e9a9548db 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -7,8 +7,9 @@ Bugfixes:
  * Code Generator: Properly skip unneeded storgae array cleanup when not reducing length.
  * Code Generator: Bugfix in modifier lookup in libraries.
  * Commandline interface: Support ``--evm-version constantinople`` properly.
- * Standard JSON: Support ``constantinople`` as ``evmVersion`` properly.
  * DocString Parser: Fix error message for empty descriptions.
+ * Standard JSON: Support ``constantinople`` as ``evmVersion`` properly.
+ * Type System: Make external library functions accessible.
 
 ### 0.4.21 (2018-03-07)
 
diff --git a/libsolidity/ast/AST.h b/libsolidity/ast/AST.h
index 863ad2fec..a25df64b7 100644
--- a/libsolidity/ast/AST.h
+++ b/libsolidity/ast/AST.h
@@ -203,6 +203,7 @@ public:
 	bool isPublic() const { return visibility() >= Visibility::Public; }
 	virtual bool isVisibleInContract() const { return visibility() != Visibility::External; }
 	bool isVisibleInDerivedContracts() const { return isVisibleInContract() && visibility() >= Visibility::Internal; }
+	bool isVisibleAsLibraryMember() const { return visibility() >= Visibility::Internal; }
 
 	std::string fullyQualifiedName() const { return sourceUnitName() + ":" + name(); }
 
diff --git a/libsolidity/ast/Types.cpp b/libsolidity/ast/Types.cpp
index c08e0e673..b2881beac 100644
--- a/libsolidity/ast/Types.cpp
+++ b/libsolidity/ast/Types.cpp
@@ -304,7 +304,7 @@ MemberList::MemberMap Type::boundFunctions(Type const& _type, ContractDefinition
 			);
 			for (FunctionDefinition const* function: library.definedFunctions())
 			{
-				if (!function->isVisibleInDerivedContracts() || seenFunctions.count(function))
+				if (!function->isVisibleAsLibraryMember() || seenFunctions.count(function))
 					continue;
 				seenFunctions.insert(function);
 				FunctionType funType(*function, false);
@@ -2875,7 +2875,7 @@ MemberList::MemberMap TypeType::nativeMembers(ContractDefinition const* _current
 		}
 		if (contract.isLibrary())
 			for (FunctionDefinition const* function: contract.definedFunctions())
-				if (function->isVisibleInDerivedContracts())
+				if (function->isVisibleAsLibraryMember())
 					members.push_back(MemberList::Member(
 						function->name(),
 						FunctionType(*function).asMemberFunction(true),
diff --git a/test/libsolidity/SolidityEndToEndTest.cpp b/test/libsolidity/SolidityEndToEndTest.cpp
index d72f3350b..44dc40f73 100644
--- a/test/libsolidity/SolidityEndToEndTest.cpp
+++ b/test/libsolidity/SolidityEndToEndTest.cpp
@@ -7024,6 +7024,21 @@ BOOST_AUTO_TEST_CASE(library_call)
 	ABI_CHECK(callContractFunction("f(uint256)", u256(33)), encodeArgs(u256(33) * 9));
 }
 
+BOOST_AUTO_TEST_CASE(library_function_external)
+{
+	char const* sourceCode = R"(
+		library Lib { function m(bytes b) external pure returns (byte) { return b[2]; } }
+		contract Test {
+			function f(bytes b) public pure returns (byte) {
+				return Lib.m(b);
+			}
+		}
+	)";
+	compileAndRun(sourceCode, 0, "Lib");
+	compileAndRun(sourceCode, 0, "Test", bytes(), map<string, Address>{{"Lib", m_contractAddress}});
+	ABI_CHECK(callContractFunction("f(bytes)", u256(0x20), u256(5), "abcde"), encodeArgs("c"));
+}
+
 BOOST_AUTO_TEST_CASE(library_stray_values)
 {
 	char const* sourceCode = R"(
diff --git a/test/libsolidity/syntaxTests/visibility/external_library_function.sol b/test/libsolidity/syntaxTests/visibility/external_library_function.sol
new file mode 100644
index 000000000..110e74dbd
--- /dev/null
+++ b/test/libsolidity/syntaxTests/visibility/external_library_function.sol
@@ -0,0 +1,14 @@
+library L {
+    function f(uint) pure external {}
+}
+
+contract C {
+    using L for *;
+
+    function f() public pure {
+        L.f(2);
+        uint x;
+        x.f();
+    }
+}
+// ----