diff --git a/faiss/IndexIVFFlat.cpp b/faiss/IndexIVFFlat.cpp
index be9e07f69..e8834e864 100644
--- a/faiss/IndexIVFFlat.cpp
+++ b/faiss/IndexIVFFlat.cpp
@@ -9,6 +9,8 @@
 
 #include <faiss/IndexIVFFlat.h>
 
+#include <omp.h>
+
 #include <cinttypes>
 #include <cstdio>
 
@@ -47,19 +49,35 @@ void IndexIVFFlat::add_core(
     direct_map.check_can_add(xids);
 
     int64_t n_add = 0;
-    for (size_t i = 0; i < n; i++) {
-        idx_t id = xids ? xids[i] : ntotal + i;
-        idx_t list_no = coarse_idx[i];
-        size_t offset;
 
-        if (list_no >= 0) {
-            const float* xi = x + i * d;
-            offset = invlists->add_entry(list_no, id, (const uint8_t*)xi);
-            n_add++;
-        } else {
-            offset = 0;
+#pragma omp parallel reduction(+ : n_add)
+    {
+        int nt = omp_get_num_threads();
+        int rank = omp_get_thread_num();
+
+        // each thread takes care of a subset of lists
+        for (size_t i = 0; i < n; i++) {
+            idx_t list_no = coarse_idx[i];
+
+            if (list_no % nt != rank) {
+                continue;
+            }
+
+            idx_t id = xids ? xids[i] : ntotal + i;
+            size_t offset;
+
+            if (list_no >= 0) {
+                const float* xi = x + i * d;
+                offset = invlists->add_entry(list_no, id, (const uint8_t*)xi);
+                n_add++;
+            } else {
+                offset = 0;
+            }
+
+#pragma omp critical
+            // executed by one thread at a time
+            direct_map.add_single_id(id, list_no, offset);
         }
-        direct_map.add_single_id(id, list_no, offset);
     }
 
     if (verbose) {
@@ -249,38 +267,50 @@ void IndexIVFFlatDedup::add_with_ids(
     quantizer->assign(na, x, idx);
 
     int64_t n_add = 0, n_dup = 0;
-    // TODO make a omp loop with this
-    for (size_t i = 0; i < na; i++) {
-        idx_t id = xids ? xids[i] : ntotal + i;
-        int64_t list_no = idx[i];
 
-        if (list_no < 0) {
-            continue;
-        }
-        const float* xi = x + i * d;
+#pragma omp parallel reduction(+ : n_add, n_dup)
+    {
+        int nt = omp_get_num_threads();
+        int rank = omp_get_thread_num();
 
-        // search if there is already an entry with that id
-        InvertedLists::ScopedCodes codes(invlists, list_no);
+        // each thread takes care of a subset of lists
+        for (size_t i = 0; i < na; i++) {
+            int64_t list_no = idx[i];
 
-        int64_t n = invlists->list_size(list_no);
-        int64_t offset = -1;
-        for (int64_t o = 0; o < n; o++) {
-            if (!memcmp(codes.get() + o * code_size, xi, code_size)) {
-                offset = o;
-                break;
+            if (list_no < 0 || list_no % nt != rank) {
+                continue;
             }
-        }
 
-        if (offset == -1) { // not found
-            invlists->add_entry(list_no, id, (const uint8_t*)xi);
-        } else {
-            // mark equivalence
-            idx_t id2 = invlists->get_single_id(list_no, offset);
-            std::pair<idx_t, idx_t> pair(id2, id);
-            instances.insert(pair);
-            n_dup++;
+            idx_t id = xids ? xids[i] : ntotal + i;
+            const float* xi = x + i * d;
+
+            // search if there is already an entry with that id
+            InvertedLists::ScopedCodes codes(invlists, list_no);
+
+            int64_t n = invlists->list_size(list_no);
+            int64_t offset = -1;
+            for (int64_t o = 0; o < n; o++) {
+                if (!memcmp(codes.get() + o * code_size, xi, code_size)) {
+                    offset = o;
+                    break;
+                }
+            }
+
+            if (offset == -1) { // not found
+                invlists->add_entry(list_no, id, (const uint8_t*)xi);
+            } else {
+                // mark equivalence
+                idx_t id2 = invlists->get_single_id(list_no, offset);
+                std::pair<idx_t, idx_t> pair(id2, id);
+
+#pragma omp critical
+                // executed by one thread at a time
+                instances.insert(pair);
+
+                n_dup++;
+            }
+            n_add++;
         }
-        n_add++;
     }
     if (verbose) {
         printf("IndexIVFFlat::add_with_ids: added %" PRId64 " / %" PRId64