X-ray topographs have been taken of gallium arsenide used for MESFET fabrication. A (100) wafer with MESFET arrays fabricated on to it contained dislocation lineage features, one of which w a s studied in detail. It was found to cause a drop of approximately 40 mV in the pinch-off voltage of the transistors it touched. The feature appeared to have an associated displacement vector of [211]type, and to be accompanied by a lattice tilt in the wafer of nearly 30 seconds of arc