diff --git a/eo-runtime/src/test/java/org/eolang/AtRhoTest.java b/eo-runtime/src/test/java/org/eolang/AtRhoTest.java
new file mode 100644
index 00000000000..de71233ff4a
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+++ b/eo-runtime/src/test/java/org/eolang/AtRhoTest.java
@@ -0,0 +1,71 @@
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2016-2024 Objectionary.com
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included
+ * in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+package org.eolang;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.atomic.AtomicInteger;
+import org.hamcrest.MatcherAssert;
+import org.hamcrest.Matchers;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+
+/**
+ * Test for {@link AtRho}.
+ *
+ * @since 0.50.0
+ */
+final class AtRhoTest {
+
+    @ParameterizedTest
+    @ValueSource(ints = {1, 4, 32})
+    void worksAsExpected(final int threads) throws InterruptedException {
+        final Attr rho = new AtRho();
+        final CountDownLatch start = new CountDownLatch(1);
+        final CountDownLatch end = new CountDownLatch(threads);
+        final AtomicInteger count = new AtomicInteger(0);
+        for (int idx = 0; idx < threads; ++idx) {
+            final Thread thread = new Thread(
+                () -> {
+                    try {
+                        start.await();
+                        final Phi phi = new Phi.ToPhi(count.incrementAndGet());
+                        rho.put(phi);
+                    } catch (final InterruptedException ex) {
+                        Thread.currentThread().interrupt();
+                    } finally {
+                        end.countDown();
+                    }
+                }
+            );
+            thread.start();
+        }
+        start.countDown();
+        end.await();
+        MatcherAssert.assertThat(
+            "The object must be equal to 1",
+            new BytesOf(rho.get().delta()).asNumber(),
+            Matchers.equalTo(1.0)
+        );
+    }
+}