diff --git a/man/rustc.1 b/man/rustc.1
index edbc6cea0266..0eaf89a560fb 100644
--- a/man/rustc.1
+++ b/man/rustc.1
@@ -264,7 +264,8 @@ which link to the standard library.
 .TP
 \fBRUST_TEST_THREADS\fR
 The test framework Rust provides executes tests in parallel. This variable sets
-the maximum number of threads used for this purpose.
+the maximum number of threads used for this purpose. This setting is overridden
+by the --test-threads option.
 
 .TP
 \fBRUST_TEST_NOCAPTURE\fR
diff --git a/src/libtest/lib.rs b/src/libtest/lib.rs
index 64515b900d5b..850127d9f295 100644
--- a/src/libtest/lib.rs
+++ b/src/libtest/lib.rs
@@ -303,6 +303,7 @@ pub struct TestOpts {
     pub nocapture: bool,
     pub color: ColorConfig,
     pub quiet: bool,
+    pub test_threads: Option<usize>,
 }
 
 impl TestOpts {
@@ -317,6 +318,7 @@ fn new() -> TestOpts {
             nocapture: false,
             color: AutoColor,
             quiet: false,
+            test_threads: None,
         }
     }
 }
@@ -334,6 +336,8 @@ fn optgroups() -> Vec<getopts::OptGroup> {
                           of stdout", "PATH"),
       getopts::optflag("", "nocapture", "don't capture stdout/stderr of each \
                                          task, allow printing directly"),
+      getopts::optopt("", "test-threads", "Number of threads used for running tests \
+                                           in parallel", "n_threads"),
       getopts::optflag("q", "quiet", "Display one character per test instead of one line"),
       getopts::optopt("", "color", "Configure coloring of output:
             auto   = colorize if stdout is a tty and tests are run on serially (default);
@@ -349,7 +353,8 @@ fn usage(binary: &str) {
 tests whose names contain the filter are run.
 
 By default, all tests are run in parallel. This can be altered with the
-RUST_TEST_THREADS environment variable when running tests (set it to 1).
+--test-threads flag or the RUST_TEST_THREADS environment variable when running
+tests (set it to 1).
 
 All tests have their standard output and standard error captured by default.
 This can be overridden with the --nocapture flag or setting RUST_TEST_NOCAPTURE
@@ -408,6 +413,18 @@ pub fn parse_opts(args: &[String]) -> Option<OptRes> {
         };
     }
 
+    let test_threads = match matches.opt_str("test-threads") {
+        Some(n_str) =>
+            match n_str.parse::<usize>() {
+                Ok(n) => Some(n),
+                Err(e) =>
+                    return Some(Err(format!("argument for --test-threads must be a number > 0 \
+                                             (error: {})", e)))
+            },
+        None =>
+            None,
+    };
+
     let color = match matches.opt_str("color").as_ref().map(|s| &**s) {
         Some("auto") | None => AutoColor,
         Some("always") => AlwaysColor,
@@ -429,6 +446,7 @@ pub fn parse_opts(args: &[String]) -> Option<OptRes> {
         nocapture: nocapture,
         color: color,
         quiet: quiet,
+        test_threads: test_threads,
     };
 
     Some(Ok(test_opts))
@@ -871,9 +889,10 @@ fn run_tests<F>(opts: &TestOpts, tests: Vec<TestDescAndFn>, mut callback: F) ->
             }
         });
 
-    // It's tempting to just spawn all the tests at once, but since we have
-    // many tests that run in other processes we would be making a big mess.
-    let concurrency = get_concurrency();
+    let concurrency = match opts.test_threads {
+        Some(n) => n,
+        None => get_concurrency(),
+    };
 
     let mut remaining = filtered_tests;
     remaining.reverse();
diff --git a/src/tools/compiletest/src/main.rs b/src/tools/compiletest/src/main.rs
index cefcc11486fe..90641b5c476d 100644
--- a/src/tools/compiletest/src/main.rs
+++ b/src/tools/compiletest/src/main.rs
@@ -310,6 +310,7 @@ pub fn test_opts(config: &Config) -> test::TestOpts {
             Err(_) => false
         },
         color: test::AutoColor,
+        test_threads: None,
     }
 }