diff --git a/src/cli/lambdapi.ml b/src/cli/lambdapi.ml
index f06916e2f..b3a92da33 100644
--- a/src/cli/lambdapi.ml
+++ b/src/cli/lambdapi.ml
@@ -114,6 +114,10 @@ let parse_cmd : Config.t -> string list -> unit = fun cfg files ->
   in
   Error.handle_exceptions run
 
+(** Running the depend mode. *)
+let depend_cmd : Config.t -> string list -> unit = fun _cfg files ->
+  Tool.Depend.print_deps files
+
 (** Possible outputs for the export command. *)
 type output = Lp | Dk | RawDk | Hrs | Xtc | RawCoq | SttCoq
 
@@ -384,6 +388,11 @@ let parse_cmd =
   Cmd.v (Cmd.info "parse" ~doc ~man:man_pkg_file)
     CLT.(const parse_cmd $ Config.full $ files)
 
+let depend_cmd =
+  let doc = "Print dependencies of the given files." in
+  Cmd.v (Cmd.info "depend" ~doc ~man:man_pkg_file)
+    CLT.(const depend_cmd $ Config.full $ files)
+
 let export_cmd =
   let doc = "Translate the given files to other formats." in
   Cmd.v (Cmd.info "export" ~doc ~man:man_pkg_file)
@@ -452,7 +461,7 @@ let _ =
   Stdlib.at_exit (Debug.print_time t0);
   Printexc.record_backtrace true;
   let cmds =
-    [ check_cmd ; parse_cmd ; export_cmd ; lsp_server_cmd
+    [ check_cmd ; parse_cmd ; depend_cmd ; export_cmd ; lsp_server_cmd
     ; decision_tree_cmd ; help_cmd ; version_cmd
     ; Init.cmd ; Install.install_cmd ; Install.uninstall_cmd
     ; index_cmd ; search_cmd ; websearch_cmd ]
diff --git a/src/lplib/string.ml b/src/lplib/string.ml
index 32e09711f..8d46ae91d 100644
--- a/src/lplib/string.ml
+++ b/src/lplib/string.ml
@@ -41,3 +41,12 @@ module B = Bytes
 let bos = B.unsafe_of_string
 let get_utf_8_uchar s i = B.get_utf_8_uchar (bos s) i
 let is_valid_utf_8 s = B.is_valid_utf_8 (bos s)
+
+let starts_with ~prefix s =
+  let len_s = length s
+  and len_pre = length prefix in
+  let rec aux i =
+    if i = len_pre then true
+    else if unsafe_get s i <> unsafe_get prefix i then false
+    else aux (i + 1)
+  in len_s >= len_pre && aux 0
diff --git a/src/parsing/package.ml b/src/parsing/package.ml
index 6e4ba5fd0..5d500e05d 100644
--- a/src/parsing/package.ml
+++ b/src/parsing/package.ml
@@ -88,3 +88,10 @@ let apply_config : string -> unit = fun fname ->
   let {root_path; _} = read cfg_file in
   let root = Filename.dirname cfg_file in
   Library.add_mapping (root_path, root)
+
+(** [root_path fname] tries to find a configuration file from [fname] and
+    return the root_path if any. *)
+let root_path fname =
+  match find_config fname with
+  | None -> None
+  | Some cfg_file -> Some (read cfg_file).root_path
diff --git a/src/tool/depend.ml b/src/tool/depend.ml
new file mode 100644
index 000000000..bf9a15768
--- /dev/null
+++ b/src/tool/depend.ml
@@ -0,0 +1,51 @@
+(** Fast extraction of the dependencies of a set of Lambdapi files without
+    parsing them. *)
+
+(* FIXME: extend to Dedukti files *)
+
+(** [string_of_file filename] returns the contents of [filename] as a
+    string *)
+let string_of_file filename =
+  let ic = Stdlib.open_in filename in
+  let n = Stdlib.in_channel_length ic in
+  let s = Bytes.create n in
+  Stdlib.really_input ic s 0 n;
+  Stdlib.close_in ic;
+  Bytes.to_string s
+
+(* FIXME: does not handle escaped names {|...|}. *)
+let re_mod = Str.regexp "\\([^ \n\t]+\\)"
+
+let search_mod root_path content =
+  print_endline ("search_mod: "^content);
+  let rec search start =
+    try let _ = Str.search_forward re_mod content start in
+        let s = Str.matched_group 1 content in
+        if Lplib.String.starts_with ~prefix:root_path s then print_endline s;
+        search (Str.match_end())
+    with _ -> ()
+  in search 0
+
+(* FIXME: does not handle comments /* ... */ between module names, or comments
+   // ... at the end of a line. *)
+let re_req =
+  Str.regexp "\\(^\\|[ \t]\\)require\\([ \n\t]+open\\)?[ \n\t]+\\([^;]+\\);"
+
+let search_req root_path content =
+  let rec search start =
+    try let _ = Str.search_forward re_req content start in
+        search_mod root_path (Str.matched_group 3 content);
+        search (Str.match_end())
+    with _ -> ()
+  in search 0
+
+let print_deps_file root_path filename =
+  search_req root_path (string_of_file filename)
+
+let print_deps = function
+  | [] -> ()
+  | (f::_) as fs ->
+      match Parsing.Package.root_path f with
+      | None -> Common.Error.fatal_no_pos "No file lambdapi.pkg found."
+      | Some root_path ->
+          List.iter (print_deps_file (String.concat "." root_path)) fs
diff --git a/src/tool/dune b/src/tool/dune
index d3cc83495..7ecdfd036 100644
--- a/src/tool/dune
+++ b/src/tool/dune
@@ -2,7 +2,7 @@
  (name tool)
  (public_name lambdapi.tool)
  (modules :standard)
- (libraries lambdapi.parsing lambdapi.core dream unix)
+ (libraries lambdapi.parsing lambdapi.core dream unix str)
  (preprocess (pps lwt_ppx)))
 
 (rule