The evolution of extreme cooperation, as found in eusocial insects (those with a worker caste), is potentially undermined by selfish reproduction among group members1, 2, 3. In some eusocial Hymenoptera (ants, bees and wasps), workers can produce male offspring from unfertilized eggs4. Kin selection theory predicts levels of worker reproduction as a function of the relatedness structure of the workers' natal colony and the colony-level costs of worker reproduction5, 6. However, the theory has been only partially successful in explaining levels of worker reproduction7, 8, 9. Here we show that workers of a eusocial bumble bee (Bombus terrestris) enter unrelated, conspecific colonies in which they then produce adult male offspring, and that such socially parasitic workers reproduce earlier and are significantly more reproductive and aggressive than resident workers that reproduce within their own colonies. Explaining levels of worker reproduction, and hence the potential of worker selfishness to undermine the evolution of cooperation, will therefore require more than simply a consideration of the kin-selected interests of resident workers. It will also require knowledge of the full set of reproductive options available to workers, including intraspecific social parasitism.