diff --git a/rugnux/RugnuxScaleMerge.cpp b/rugnux/RugnuxScaleMerge.cpp
index 45a20da2b..a36205b67 100644
--- a/rugnux/RugnuxScaleMerge.cpp
+++ b/rugnux/RugnuxScaleMerge.cpp
@@ -602,13 +602,16 @@ bool Rugnux::ScaleMergeAndSymmetry(PipelineLocals &p) {
     const auto reseat_integrated = [&](const gemmi::Op &cob) {
         const gemmi::Mat33 basis = BasisMatrix(cob);
         const gemmi::Op hkl = HklOperator(cob);
-        for (auto &io : indexer->GetIntegrationOutcome()) {
+        // One image per task: each touches only its own outcome.
+        auto &outcomes = indexer->GetIntegrationOutcome();
+        ParallelFor(static_cast<int>(outcomes.size()), config_.nthreads, [&](int o) {
+            auto &io = outcomes[o];
             io.latt = io.latt.Multiply(basis);
             for (auto &r : io.reflections) {
                 const gemmi::Op::Miller h = hkl.apply_to_hkl({{r.h, r.k, r.l}});
                 r.h = h[0]; r.k = h[1]; r.l = h[2];
             }
-        }
+        });
         result.consensus_cell = CellInBasis(*result.consensus_cell, cob);
         end_msg.unit_cell = result.consensus_cell;
         if (end_msg.rotation_lattice.has_value())