diff --git a/src/sortbin.cpp b/src/sortbin.cpp
index 23935e5..2c32806 100644
--- a/src/sortbin.cpp
+++ b/src/sortbin.cpp
@@ -1342,16 +1342,28 @@ void quicksort_records_parallel(
 }
 
 
-/** Sort the specified block of records (in-place). */
+/**
+ * Sort the specified block of records (in-place).
+ *
+ * Optionally use a thread pool for parallel sorting.
+ */
 void sort_records(
     unsigned char * buffer,
     size_t record_size,
-    size_t num_records)
+    size_t num_records,
+    unsigned int num_threads,
+    ThreadPool * thread_pool)
 {
-    // TODO : multi-threaded quicksort
-
-//    heap_sort_records(buffer, record_size, num_records);
-    quicksort_records(buffer, record_size, num_records);
+    if (num_threads > 1 && thread_pool != NULL) {
+        quicksort_records_parallel(
+            buffer,
+            record_size,
+            num_records,
+            num_threads,
+            thread_pool);
+    } else {
+        quicksort_records(buffer, record_size, num_records);
+    }
 }
 
 
@@ -1430,7 +1442,7 @@ void single_pass(
     timer.stop();
     log(ctx, "  t = %.3f seconds\n", timer.value());
 
-    // Set up thread pool.
+    // Create thread pool for parallel sorting.
     std::unique_ptr<ThreadPool> thread_pool;
     if (ctx.num_threads > 1) {
         log(ctx, "creating thread pool with %u threads\n", ctx.num_threads);
@@ -1440,16 +1452,12 @@ void single_pass(
     // Sort records in memory buffer.
     log(ctx, "sorting records using %u threads\n", ctx.num_threads);
     timer.start();
-    if (ctx.num_threads > 1) {
-        quicksort_records_parallel(
-            buffer.data(),
-            ctx.record_size,
-            num_records,
-            ctx.num_threads,
-            thread_pool.get());
-    } else {
-        quicksort_records(buffer.data(), ctx.record_size, num_records);
-    }
+    sort_records(
+        buffer.data(),
+        ctx.record_size,
+        num_records,
+        ctx.num_threads,
+        thread_pool.get());
     timer.stop();
     log(ctx, "  t = %.3f seconds\n", timer.value());
 
@@ -1517,7 +1525,12 @@ void sort_pass(
     assert(records_per_block < SIZE_MAX / record_size);
     std::vector<unsigned char> buffer(records_per_block * record_size);
 
-// TODO : multi-threaded sorting with thread pool
+    // Create thread pool for parallel sorting.
+    std::unique_ptr<ThreadPool> thread_pool;
+    if (ctx.num_threads > 1) {
+        log(ctx, "creating thread pool with %u threads\n", ctx.num_threads);
+        thread_pool.reset(new ThreadPool(ctx.num_threads));
+    }
 
     // Loop over blocks to be sorted.
     for (uint64_t block_index = 0; block_index < num_blocks; block_index++) {
@@ -1542,7 +1555,9 @@ void sort_pass(
         sort_records(
             buffer.data(),
             record_size,
-            block_num_records);
+            block_num_records,
+            ctx.num_threads,
+            thread_pool.get());
 
         // Write block.
         BinaryFile& output_file =