diff --git a/src/main/java/io/reactivex/internal/operators/flowable/FlowableFlatMapMaybe.java b/src/main/java/io/reactivex/internal/operators/flowable/FlowableFlatMapMaybe.java
index ea00d4df2c..74413e85d3 100644
--- a/src/main/java/io/reactivex/internal/operators/flowable/FlowableFlatMapMaybe.java
+++ b/src/main/java/io/reactivex/internal/operators/flowable/FlowableFlatMapMaybe.java
@@ -191,14 +191,18 @@ void innerSuccess(InnerObserver inner, R value) {
                     }
                 } else {
                     SpscLinkedArrayQueue<R> q = getOrCreateQueue();
-                    q.offer(value);
+                    synchronized (q) {
+                        q.offer(value);
+                    }
                 }
                 if (decrementAndGet() == 0) {
                     return;
                 }
             } else {
                 SpscLinkedArrayQueue<R> q = getOrCreateQueue();
-                q.offer(value);
+                synchronized (q) {
+                    q.offer(value);
+                }
                 active.decrementAndGet();
                 if (getAndIncrement() != 0) {
                     return;
diff --git a/src/main/java/io/reactivex/internal/operators/flowable/FlowableFlatMapSingle.java b/src/main/java/io/reactivex/internal/operators/flowable/FlowableFlatMapSingle.java
index 9b9dbc8d89..434d36d01f 100644
--- a/src/main/java/io/reactivex/internal/operators/flowable/FlowableFlatMapSingle.java
+++ b/src/main/java/io/reactivex/internal/operators/flowable/FlowableFlatMapSingle.java
@@ -191,14 +191,18 @@ void innerSuccess(InnerObserver inner, R value) {
                     }
                 } else {
                     SpscLinkedArrayQueue<R> q = getOrCreateQueue();
-                    q.offer(value);
+                    synchronized (q) {
+                        q.offer(value);
+                    }
                 }
                 if (decrementAndGet() == 0) {
                     return;
                 }
             } else {
                 SpscLinkedArrayQueue<R> q = getOrCreateQueue();
-                q.offer(value);
+                synchronized (q) {
+                    q.offer(value);
+                }
                 active.decrementAndGet();
                 if (getAndIncrement() != 0) {
                     return;
diff --git a/src/main/java/io/reactivex/internal/operators/observable/ObservableFlatMapMaybe.java b/src/main/java/io/reactivex/internal/operators/observable/ObservableFlatMapMaybe.java
index 0de37223ac..28a8547670 100644
--- a/src/main/java/io/reactivex/internal/operators/observable/ObservableFlatMapMaybe.java
+++ b/src/main/java/io/reactivex/internal/operators/observable/ObservableFlatMapMaybe.java
@@ -167,7 +167,9 @@ void innerSuccess(InnerObserver inner, R value) {
                 }
             } else {
                 SpscLinkedArrayQueue<R> q = getOrCreateQueue();
-                q.offer(value);
+                synchronized (q) {
+                    q.offer(value);
+                }
                 active.decrementAndGet();
                 if (getAndIncrement() != 0) {
                     return;
diff --git a/src/main/java/io/reactivex/internal/operators/observable/ObservableFlatMapSingle.java b/src/main/java/io/reactivex/internal/operators/observable/ObservableFlatMapSingle.java
index 3b8c2cc2cd..8fb7e63813 100644
--- a/src/main/java/io/reactivex/internal/operators/observable/ObservableFlatMapSingle.java
+++ b/src/main/java/io/reactivex/internal/operators/observable/ObservableFlatMapSingle.java
@@ -167,7 +167,9 @@ void innerSuccess(InnerObserver inner, R value) {
                 }
             } else {
                 SpscLinkedArrayQueue<R> q = getOrCreateQueue();
-                q.offer(value);
+                synchronized (q) {
+                    q.offer(value);
+                }
                 active.decrementAndGet();
                 if (getAndIncrement() != 0) {
                     return;