diff --git a/image_analysis/WriteReflections.cpp b/image_analysis/WriteReflections.cpp
index e1c1bce98..faf3b5075 100644
--- a/image_analysis/WriteReflections.cpp
+++ b/image_analysis/WriteReflections.cpp
@@ -1033,6 +1033,15 @@ gemmi::Mtz UnmergedMtz(const std::vector<IntegrationOutcome> &outcomes,
     // removing them turns its test into an assumption. XDS and DIALS draw the line in the same place.
     const char centering = mtz.spacegroup ? mtz.spacegroup->hm[0] : 'P';
 
+    // On stills only the crystals the merge takes are written: one whose cell is not the run's was indexed
+    // in another lattice (on few spots, typically an axis multiple), so its indices do not name the
+    // reflections they would be read as. A rotation run has the one lattice.
+    const auto &indexing = experiment.GetIndexingSettings();
+    const auto merged_crystal = [&](const IntegrationOutcome &outcome) {
+        return scanning || outcome.latt.GetUnitCell().is_close(unitCell, indexing.GetUnitCellDistTolerance(),
+                                                               indexing.GetUnitCellAngleTolerance_deg());
+    };
+
     std::vector<std::vector<Reflection>> fulls;
     if (scanning && sum_partials)
         fulls = SumRockingEvents(outcomes,
@@ -1057,8 +1066,9 @@ gemmi::Mtz UnmergedMtz(const std::vector<IntegrationOutcome> &outcomes,
                 add_row(r);
     } else {
         for (const auto &outcome : outcomes)
-            for (const auto &r : outcome.reflections)
-                add_row(r);
+            if (merged_crystal(outcome))
+                for (const auto &r : outcome.reflections)
+                    add_row(r);
     }
 
     // Every row has its own slot, so they are filled on several workers.
@@ -1104,7 +1114,7 @@ gemmi::Mtz UnmergedMtz(const std::vector<IntegrationOutcome> &outcomes,
     // the sweep, which is the one matrix POINTLESS also writes into every batch.
     std::optional<CrystalLattice> lattice_at_zero;
     for (const auto &outcome : outcomes) {
-        if (outcome.reflections.empty() || outcome.latt.CalcVolume() <= 1.0f)
+        if (outcome.reflections.empty() || outcome.latt.CalcVolume() <= 1.0f || !merged_crystal(outcome))
             continue;
         const float mid_deg = phi_start_deg(outcome.reflections.front().image_number) + wedge_deg / 2.0f;
         lattice_at_zero = gon ? outcome.latt.Multiply(gon->GetTransformationAngle(mid_deg)) : outcome.latt;
diff --git a/tests/WriteReflectionsTest.cpp b/tests/WriteReflectionsTest.cpp
index 848a3d02c..51512f4ec 100644
--- a/tests/WriteReflectionsTest.cpp
+++ b/tests/WriteReflectionsTest.cpp
@@ -335,3 +335,29 @@ TEST_CASE("MTZ history: command line and input, in records of at most 80 charact
     CHECK(joined.find("Input file: " + provenance.input_identity) != std::string::npos);
     CHECK(mtz.history.size() <= 30);
 }
+
+TEST_CASE("Unmerged MTZ: on stills, only the crystals on the run's cell", "[write_reflections][portable]") {
+    // A still indexed with its c axis doubled names other reflections by the same indices, so its rows
+    // would be read as the wrong reflections; it is left out, the crystal on the run's cell is written.
+    const auto experiment = TestExperiment();   // no goniometer: stills
+    std::vector<IntegrationOutcome> outcomes(2);
+    outcomes[0].latt = CrystalLattice(TETRAGONAL_CELL);
+    outcomes[1].latt = CrystalLattice(UnitCell{47.0f, 47.0f, 126.0f, 90.0f, 90.0f, 90.0f});
+    for (int f = 0; f < 2; ++f)
+        for (int l = 1; l <= 5; ++l) {
+            Reflection r{};
+            r.h = 1; r.k = 2; r.l = l;
+            r.image_number = static_cast<float>(f);
+            r.I = 100.0f;
+            r.sigma = 10.0f;
+            r.prescaling_corr = 1.0f;
+            r.partiality = 1.0f;
+            outcomes[f].reflections.push_back(r);
+        }
+    const gemmi::Mtz mtz = UnmergedMtz(outcomes, TETRAGONAL_CELL, experiment, false, OutputProvenance{}, 1);
+    CHECK(mtz.nreflections == 5);
+    const auto batch = mtz.column_with_label("BATCH");
+    REQUIRE(batch != nullptr);
+    for (int i = 0; i < mtz.nreflections; ++i)
+        CHECK((*batch)[i] == 1.0f);
+}