Psithyrus spp. (Hymenoptera: Apidae) are obligate social parasites of bumble bees (Bombus spp.). Morphological and physiological features of P. ashtoni (Cresson) females, which may facilitate the successful usurpation of host nests, were examined. Parasite females were no larger than B. terricola Kirby queens, although they possessed a number of protective modifications to the exoskeleton, as well as increased offensive armament, including larger mandibles and a longer venom gland. Females of P. ashtoni and P. insularis (Smith) possessed a greater number of ovarioles than host queens, and produced smaller eggs. The Dufour’s glands of the two Psithyrus species were significantly larger than those of B. terricola. There appears to have been considerable convergence in the evolution of socially parasitic lifestyles in bumble bees and wasps (Vespula spp.), presumably as a consequence of similar selective pressures operating on parasites belonging to these distantly related but socially similar taxa.