In many species of animals, females typically mate with more than one male (polyandry). Some social insects carry this behaviour to extremes(1). For example, honeybee queens mate with ten to twenty (car even more) males on their nuptial flights(2). The reasons for this behaviour remain unknown, given the obvious costs of time, energy and exposure to predation. Several potential benefits of polyandry have been proposed(1,3,4), but none are well supported yet. Here we test the hypothesis that genetic diversity among a female＇s offspring may offer some protection from parasitism(5-7). We artificially inseminated queens of a bumble-bee (Bombus terrestris L.) with sperm of either low or high genetic diversity. The resulting colonies were exposed to parasitism under field conditions. High-diversity colonies had fewer parasites and showed greater reproductive success, on average, than did low-diversity colonies. We suggest that female mating frequency may be influenced in part by parasites.