diff -r 4b99e8f05b44 src/wifi/model/dca-txop.cc
--- a/src/wifi/model/dca-txop.cc	Thu Jul 02 21:11:24 2015 +0200
+++ b/src/wifi/model/dca-txop.cc	Mon Jul 06 19:16:41 2015 +0200
@@ -445,6 +445,7 @@
       NS_ASSERT (m_currentPacket != 0);
       uint16_t sequence = m_txMiddle->GetNextSequenceNumberfor (&m_currentHdr);
       m_currentHdr.SetSequenceNumber (sequence);
+      m_stationManager->UpdateFragmentationThreshold ();
       m_currentHdr.SetFragmentNumber (0);
       m_currentHdr.SetNoMoreFragments ();
       m_currentHdr.SetNoRetry ();
diff -r 4b99e8f05b44 src/wifi/model/edca-txop-n.cc
--- a/src/wifi/model/edca-txop-n.cc	Thu Jul 02 21:11:24 2015 +0200
+++ b/src/wifi/model/edca-txop-n.cc	Mon Jul 06 19:16:41 2015 +0200
@@ -506,6 +506,7 @@
 
           uint16_t sequence = m_txMiddle->GetNextSequenceNumberfor (&m_currentHdr);
           m_currentHdr.SetSequenceNumber (sequence);
+          m_stationManager->UpdateFragmentationThreshold ();
           m_currentHdr.SetFragmentNumber (0);
           m_currentHdr.SetNoMoreFragments ();
           m_currentHdr.SetNoRetry ();
diff -r 4b99e8f05b44 src/wifi/model/wifi-remote-station-manager.cc
--- a/src/wifi/model/wifi-remote-station-manager.cc	Thu Jul 02 21:11:24 2015 +0200
+++ b/src/wifi/model/wifi-remote-station-manager.cc	Mon Jul 06 19:16:41 2015 +0200
@@ -861,7 +861,7 @@
        * ASN.1 encoding of the MAC and PHY MIB (256 ... 8000)
        */
       NS_LOG_WARN ("Fragmentation threshold should be larger than 256. Setting to 256.");
-      m_fragmentationThreshold = 256;
+      m_nextFragmentationThreshold = 256;
     }
   else
     {
@@ -872,15 +872,21 @@
       if (threshold % 2 != 0)
         {
           NS_LOG_WARN ("Fragmentation threshold should be an even number. Setting to " << threshold - 1);
-          m_fragmentationThreshold = threshold - 1;
+          m_nextFragmentationThreshold = threshold - 1;
         }
       else
         {
-          m_fragmentationThreshold = threshold;
+          m_nextFragmentationThreshold = threshold;
         }
     }
 }
 
+void
+WifiRemoteStationManager::UpdateFragmentationThreshold (void)
+{
+  m_fragmentationThreshold = m_nextFragmentationThreshold;
+}
+
 uint32_t
 WifiRemoteStationManager::DoGetFragmentationThreshold (void) const
 {
diff -r 4b99e8f05b44 src/wifi/model/wifi-remote-station-manager.h
--- a/src/wifi/model/wifi-remote-station-manager.h	Thu Jul 02 21:11:24 2015 +0200
+++ b/src/wifi/model/wifi-remote-station-manager.h	Mon Jul 06 19:16:41 2015 +0200
@@ -157,6 +157,11 @@
    */
   void SetFragmentationThreshold (uint32_t threshold);
   /**
+   * Typically called to update the fragmentation threshold at the start of a new transmission.
+   * This avoid that the fragmentation threshold gets changed during a transmission (see bug 730).
+   */
+  void UpdateFragmentationThreshold (void);
+  /**
    * Records HT capabilities of the remote station.
    *
    * \param from the address of the station being recorded
@@ -990,7 +995,8 @@
   uint32_t m_maxSsrc;  //!< Maximum STA short retry count (SSRC)
   uint32_t m_maxSlrc;  //!< Maximum STA long retry count (SLRC)
   uint32_t m_rtsCtsThreshold;  //!< Threshold for RTS/CTS
-  uint32_t m_fragmentationThreshold;  //!< Threshold for fragmentation
+  uint32_t m_fragmentationThreshold;  //!< Current threshold for fragmentation
+  uint32_t m_nextFragmentationThreshold;  //!< Threshold for fragmentation that will be used for the next transmission
   uint8_t m_defaultTxPowerLevel;  //!< Default tranmission power level
   WifiMode m_nonUnicastMode;  //!< Transmission mode for non-unicast DATA frames