diff --git a/src/SimpleLog.cxx b/src/SimpleLog.cxx
index 15e5fc4..f0afd95 100644
--- a/src/SimpleLog.cxx
+++ b/src/SimpleLog.cxx
@@ -21,6 +21,7 @@
 #include <string.h>
 #include <algorithm>
 #include <string>
+#include <mutex>
 
 class SimpleLog::Impl
 {
@@ -60,6 +61,7 @@ class SimpleLog::Impl
   void rotate(); // this renames older files
 
   friend class SimpleLog;
+  std::mutex fileMutex;
 };
 
 SimpleLog::Impl::Impl()
@@ -140,6 +142,9 @@ int SimpleLog::Impl::logV(SimpleLog::Impl::Severity s, const char* message, va_l
   ix++;
   buffer[ix] = 0;
 
+  int nBytes = 0; // count bytes output
+  {
+  std::lock_guard<std::mutex> lock(fileMutex);
   int fd;
   if (fp != NULL) {
     if ((ix + logFileSize > rotateMaxBytes) && (rotateMaxBytes > 0)) {
@@ -161,10 +166,11 @@ int SimpleLog::Impl::logV(SimpleLog::Impl::Severity s, const char* message, va_l
       fd = fdStdout;
     }
   }
-  int nBytes = write(fd, buffer, ix);
+  nBytes = write(fd, buffer, ix);
   if ((fp != NULL) && (nBytes > 0)) {
     logFileSize += nBytes;
   }
+  }
   if (nBytes != (int)ix) {
     return -1;
   }
diff --git a/test/testSimpleLog.cxx b/test/testSimpleLog.cxx
index 74fd1f6..d274290 100644
--- a/test/testSimpleLog.cxx
+++ b/test/testSimpleLog.cxx
@@ -15,6 +15,14 @@
 
 #include <Common/SimpleLog.h>
 #include <unistd.h>
+#include <thread>
+#include <vector>
+
+void logWorker(SimpleLog& log, int threadId, int count) {
+  for (int i = 0; i < count; ++i) {
+    log.info("Thread %d - test message %d", threadId, i);
+  }
+}
 
 int main()
 {
@@ -25,5 +33,18 @@ int main()
     theLog.info("test message %d", i);
   }
   // sleep(10);
+
+  // test parallel threads
+  theLog.setLogFile("/tmp/testthread.log", 1000, 10, 0);
+  const int numThreads = 10;
+  const int messagesPerThread = 10;
+  std::vector<std::thread> threads;
+  for (int t = 0; t < numThreads; ++t) {
+    threads.emplace_back(logWorker, std::ref(theLog), t, messagesPerThread);
+  }
+  for (auto& th : threads) {
+    th.join();
+  }
+
   return 0;
 }