The nature and origin of the defects responsible for the rapid degradation of stripe geometry GaAs&sngbnd;GaAlAs double‐heterostructure lasers have been identified by transmission electron microscopy. These defects are formed by a three‐dimensional dislocation network which originates at a dislocation crossing the GaAlAs and GaAs epilayers. The propagation of the dislocation network takes place by aclimbmechanism induced by the operation of the device.