diff --git a/book/src/workspaces.md b/book/src/workspaces.md
index b70cf40d..d049ec68 100644
--- a/book/src/workspaces.md
+++ b/book/src/workspaces.md
@@ -2,11 +2,12 @@
 
 cargo-mutants supports testing Cargo workspaces that contain multiple packages. The entire workspace tree is copied.
 
-By default, all source files in all packages in the workspace are tested.
+By default, cargo-mutants has [the same behavior as Cargo](https://doc.rust-lang.org/cargo/reference/workspaces.html):
 
-**NOTE: This behavior is likely to change in future: see <https://github.com/sourcefrog/cargo-mutants/issues/156>.**
-
-With the `--package` option, only mutants from the package with the given name are testeg. The effect can be seen in `--list`, etc. This option can be repeated.
+* If `--workspace` is given, all packages in the workspace are tested.
+* If `--package` is given, the named packages are tested.
+* If the starting directory (or `-d` directory) is in a package, that package is tested.
+* Otherwise, the starting directory must be in a virtual workspace. If it specifies default members, they are tested. Otherwise, all packages are tested.
 
 You can use the `--file` options to restrict cargo-mutants to testing only files
 from some subdirectory, e.g. with `-f "utils/**/*.rs"`. (Remember to quote globs
diff --git a/src/main.rs b/src/main.rs
index 17b8d202..dafe78b8 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -191,6 +191,10 @@ struct Args {
     #[arg(long, action = clap::ArgAction::SetTrue)]
     version: bool,
 
+    /// test every package in the workspace.
+    #[arg(long)]
+    workspace: bool,
+
     /// additional args for all cargo invocations.
     #[arg(long, short = 'C', allow_hyphen_values = true)]
     cargo_arg: Vec<String>,
@@ -237,8 +241,9 @@ fn main() -> Result<()> {
     debug!(?options);
     let package_filter = if !args.mutate_packages.is_empty() {
         PackageFilter::explicit(&args.mutate_packages)
+    } else if args.workspace {
+        PackageFilter::All
     } else {
-        // TODO: --workspace to ::All
         PackageFilter::Auto(start_dir.to_owned())
     };
     let discovered = workspace.discover(&package_filter, &options, &console)?;
diff --git a/src/workspace.rs b/src/workspace.rs
index e9147d82..27fa05d3 100644
--- a/src/workspace.rs
+++ b/src/workspace.rs
@@ -350,10 +350,38 @@ mod test {
         );
     }
 
+    #[test]
+    fn package_filter_all_from_subdir_gets_everything() {
+        let subdir_path = Utf8Path::new("testdata/tree/workspace/main");
+        let workspace = Workspace::open(subdir_path).expect("Find workspace root");
+        let packages = workspace.packages(&PackageFilter::All).unwrap();
+        assert_eq!(
+            packages.iter().map(|p| &p.name).collect_vec(),
+            ["cargo_mutants_testdata_workspace_utils", "main", "main2"]
+        );
+    }
+
     #[test]
     fn auto_packages_in_workspace_subdir_finds_single_package() {
-        let workspace =
-            Workspace::open("testdata/tree/workspace/main").expect("Find workspace root");
+        let subdir_path = Utf8Path::new("testdata/tree/workspace/main");
+        let workspace = Workspace::open(subdir_path).expect("Find workspace root");
+        let packages = workspace
+            .packages(&PackageFilter::Auto(subdir_path.to_owned()))
+            .unwrap();
+        assert_eq!(packages.iter().map(|p| &p.name).collect_vec(), ["main"]);
+    }
+
+    #[test]
+    fn auto_packages_in_virtual_workspace_gets_everything() {
+        let path = Utf8Path::new("testdata/tree/workspace");
+        let workspace = Workspace::open(path).expect("Find workspace root");
+        let packages = workspace
+            .packages(&PackageFilter::Auto(path.to_owned()))
+            .unwrap();
+        assert_eq!(
+            packages.iter().map(|p| &p.name).collect_vec(),
+            ["cargo_mutants_testdata_workspace_utils", "main", "main2"]
+        );
     }
 
     #[test]
diff --git a/tests/cli/workspace.rs b/tests/cli/workspace.rs
index 3765d037..4393f65a 100644
--- a/tests/cli/workspace.rs
+++ b/tests/cli/workspace.rs
@@ -39,7 +39,7 @@ fn list_files_json_workspace() {
 #[test]
 fn list_files_as_json_in_workspace_subdir() {
     run()
-        .args(["mutants", "--list-files", "--json"])
+        .args(["mutants", "--list-files", "--json", "--workspace"])
         .current_dir("testdata/tree/workspace/main2")
         .assert()
         .stdout(indoc! {r#"