diff --git a/starlark/parser.h b/starlark/parser.h
index 0e2d5cd..177d410 100644
--- a/starlark/parser.h
+++ b/starlark/parser.h
@@ -347,6 +347,7 @@ class Parser {
 
     std::optional<std::vector<Argument>> parse_argument_list() {
         std::vector<Argument> args;
+        bool seen_kw_arg = false;
 
         while (true) {
             auto maybe_token = next_token();
@@ -391,6 +392,7 @@ class Parser {
                 }
 
                 if (std::holds_alternative<token::Equals>(*next)) {
+                    seen_kw_arg = true;
                     name = Identifier{.name = std::move(ident->name)};
 
                     auto value_token = next_token();
@@ -414,6 +416,11 @@ class Parser {
                 reconsume(std::move(*next));
             }
 
+            if (seen_kw_arg) {
+                std::cerr << "Positional argument may not follow keyword argument.\n";
+                return std::nullopt;
+            }
+
             auto value_expr = parse_expression(token);
             if (!value_expr) {
                 std::cerr << "Failed to parse expression for argument value.\n";
diff --git a/starlark/parser_test.cc b/starlark/parser_test.cc
index 1fa44da..9117f7f 100644
--- a/starlark/parser_test.cc
+++ b/starlark/parser_test.cc
@@ -65,7 +65,7 @@ int main() {
             },
         },
         {
-            R"(foo(bar = "baz", "qux"))",
+            R"(foo("qux", bar = "baz"))",
             starlark::Program{
                 .statements{
                     starlark::ExpressionStmt{
@@ -74,14 +74,14 @@ int main() {
                                 .target = std::make_shared<starlark::Expression>(
                                     starlark::Identifier{"foo"}),
                                 .args{
-                                    {
-                                        starlark::Identifier{"bar"},
-                                        starlark::StringLiteral{"baz"},
-                                    },
                                     {
                                         std::nullopt,
                                         starlark::StringLiteral{"qux"},
                                     },
+                                    {
+                                        starlark::Identifier{"bar"},
+                                        starlark::StringLiteral{"baz"},
+                                    },
                                 },
                             },
                         },
@@ -90,7 +90,7 @@ int main() {
             },
         },
         {
-            R"(foo(bar = baz, qux()))",
+            R"(foo(qux(), bar = baz))",
             starlark::Program{
                 .statements{
                     starlark::ExpressionStmt{
@@ -99,10 +99,6 @@ int main() {
                                 .target = std::make_shared<starlark::Expression>(
                                     starlark::Identifier{"foo"}),
                                 .args{
-                                    {
-                                        starlark::Identifier{"bar"},
-                                        starlark::Identifier{"baz"},
-                                    },
                                     {
                                         std::nullopt,
                                         starlark::CallExpr{
@@ -111,6 +107,10 @@ int main() {
                                             .args{},
                                         },
                                     },
+                                    {
+                                        starlark::Identifier{"bar"},
+                                        starlark::Identifier{"baz"},
+                                    },
                                 },
                             },
                         },
@@ -331,7 +331,9 @@ int main() {
     static constexpr auto kExpectedParseFailures = std::to_array<std::string_view>({
         // CallExpr
         // Missing closing parenthesis.
-        R"(foo(bar = "baz", "qux")",
+        R"(foo("qux")",
+        // Positional argument after kw argument.
+        R"(foo(bar = baz, qux()))",
 
         // ListExpr
         // Tokenization error.