[110] lattice images of dissociated and undissociated 60[ddot] dislocations in semiinsulating GaAs have been obtained using high-resolution transmission electron microscopy. The majority of dislocations (approximately 80%) observed end-on in the samples which were plastically deformed at 415[ddot]C are dissociated 60[ddot] dislocations, but a significant number of undissociated dislocations were also found. Image simulations were carried out for the 30[ddot] and 90[ddot] partials. It could be shown that dissociated 60[ddot] dislocations are present in the glide set configuration by simulating the 30[ddot] glide and 30[ddot] shuffle set partial and matching simulated and experimental images. © 1989 Taylor & Francis Group, LLC.