Inbreeding depression has detrimental effects on many organisms, but its effects are potentially greater in organisms that have at least one asexually reproducing life stage. Here, the existence of severe inbreeding depression upon selfing (r = 1) in the cyclic parthenogenetic aphid Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae) is documented. Egg hatching success and offspring survival of inbred mating pairs are significantly lower than that of outbred mating pairs. Two possible mechanisms for avoiding selfing are examined: avoidance of partners of identical genetic makeup and avoidance of partners of the same body color (as a proxy for genetic similarity). Mating between males and females of the same color was as successful as mating between partners of different colors. In contrast, the success of mating between close kin was consistently reduced compared to that of mating between genetically unrelated partners. Interestingly, mating between close kin proceeded normally until the very last stage of the mating process. Thus, inbreeding avoidance appears to take place sometime between copulation and sperm transfer, suggesting that cryptic female choice may play a role in the process.