diff --git a/src/bin/cli.ml b/src/bin/cli.ml
index 1118ac02c..7e0606311 100644
--- a/src/bin/cli.ml
+++ b/src/bin/cli.ml
@@ -68,9 +68,9 @@ let syntax =
   let open Arg in
   let info =
     let docv = "SYNTAX" in
-    let doc = "$(docv) is the syntax that will be used. Currently supported syntaxes are \"pascaligo\" and \"cameligo\". \"pascaligo\" is the default." in
+    let doc = "$(docv) is the syntax that will be used. Currently supported syntaxes are \"pascaligo\" and \"cameligo\". By default, the syntax is guessed from the extension (.ligo and .mligo, respectively)." in
     info ~docv ~doc ["syntax" ; "s"] in
-  value @@ opt string "pascaligo" info
+  value @@ opt string "auto" info
 
 let compile_file =
   let f source entry_point syntax =
diff --git a/src/main/run_source.ml b/src/main/run_source.ml
index 79f71ce97..e97d9d855 100644
--- a/src/main/run_source.ml
+++ b/src/main/run_source.ml
@@ -95,7 +95,22 @@ let parsify_expression_ligodity = fun source ->
     Simplify.Ligodity.simpl_expression raw in
   ok simplified
 
+let detect_syntax = fun syntax source ->
+  if String.equal syntax "auto" then
+    begin
+      let subr s n =
+        String.sub s (String.length s - n) n in
+      if String.equal (subr source 5) ".ligo"
+      then ok "pascaligo"
+      else if String.equal (subr source 6) ".mligo"
+      then ok "cameligo"
+      else simple_fail "cannot auto-detect syntax, pleas use -s name_of_syntax"
+    end
+  else
+    ok syntax
+
 let parsify = fun syntax source ->
+  let%bind syntax = detect_syntax syntax source in
   let%bind parsify = match syntax with
     | "pascaligo" -> ok parsify_pascaligo
     | "cameligo" -> ok parsify_ligodity
@@ -104,6 +119,7 @@ let parsify = fun syntax source ->
   parsify source
 
 let parsify_expression = fun syntax source ->
+  let%bind syntax = detect_syntax syntax source in
   let%bind parsify = match syntax with
     | "pascaligo" -> ok parsify_expression_pascaligo
     | "cameligo" -> ok parsify_expression_ligodity