diff --git a/zebra-chain/src/block/tests.rs b/zebra-chain/src/block/tests.rs
index eb6fd931..df708aee 100644
--- a/zebra-chain/src/block/tests.rs
+++ b/zebra-chain/src/block/tests.rs
@@ -15,6 +15,7 @@ use proptest::{
     prelude::*,
     test_runner::Config,
 };
+use std::env;
 use std::io::{Cursor, ErrorKind, Write};
 
 impl Arbitrary for BlockHeader {
@@ -214,8 +215,12 @@ proptest! {
 }
 
 proptest! {
-    // The block roundtrip test can be really slow
-    #![proptest_config(Config::with_cases(16))]
+    // The block roundtrip test can be really slow, so we use fewer cases by
+    // default. Set the PROPTEST_CASES env var to override this default.
+    #![proptest_config(Config::with_cases(env::var("PROPTEST_CASES")
+                                          .ok()
+                                          .and_then(|v| v.parse().ok())
+                                          .unwrap_or(16)))]
 
     #[test]
     fn block_roundtrip(block in any::<Block>()) {