diff --git a/WORKSPACE b/WORKSPACE
index 0470bc042..9a3a1c138 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -20,6 +20,7 @@ nixpkgs_git_repository(
 nixpkgs_local_repository(
     name = "nixpkgs",
     nix_file = "//:nixpkgs.nix",
+    nix_file_deps = ["//:nixpkgs.json"],
 )
 
 nixpkgs_package(
@@ -39,6 +40,7 @@ nixpkgs_package(
     nix_file_content = "let pkgs = import <nixpkgs> { config = {}; overlays = []; }; in pkgs.hello",
     # Deliberately not @nixpkgs, to test whether explict file works.
     repositories = {"nixpkgs": "//:nixpkgs.nix"},
+    nix_file_deps = ["//:nixpkgs.json"],
 )
 
 nixpkgs_package(
@@ -101,6 +103,7 @@ nixpkgs_package(
     attribute_path = "hello",
     nix_file = "//tests:relative_imports.nix",
     nix_file_deps = [
+        "//:nixpkgs.json",
         "//:nixpkgs.nix",
         "//tests:relative_imports/nixpkgs.nix",
     ],
diff --git a/nixpkgs.json b/nixpkgs.json
new file mode 100644
index 000000000..8716771ac
--- /dev/null
+++ b/nixpkgs.json
@@ -0,0 +1,8 @@
+{
+  "owner": "NixOS",
+  "repo": "nixpkgs-channels",
+  "branch": "nixpkgs-unstable",
+  "rev": "0620e0fdbf4d79df771afd28f741b2159f381d2b",
+  "sha256": "046l2c83s568c306hnm8nfdpdhmgnbzgid354hr7p0khq3jx3lhf"
+}
+
diff --git a/nixpkgs.nix b/nixpkgs.nix
index 2b7d682d5..3425ec91b 100644
--- a/nixpkgs.nix
+++ b/nixpkgs.nix
@@ -1,10 +1,9 @@
 let
   # nixpkgs-unstable as of 2019-04-25
-  nixpkgsRev = "0620e0fdbf4";
-  nixpkgsSha256 = "046l2c83s568c306hnm8nfdpdhmgnbzgid354hr7p0khq3jx3lhf";
+  spec = builtins.fromJSON (builtins.readFile ./nixpkgs.json);
   nixpkgs = fetchTarball {
-    url = "https://github.com/nixos/nixpkgs/archive/${nixpkgsRev}.tar.gz";
-    sha256 = nixpkgsSha256;
+    url = "https://github.com/${spec.owner}/${spec.repo}/archive/${spec.rev}.tar.gz";
+    sha256 = spec.sha256;
   };
 in
 import nixpkgs