IN most species of social Hymenoptera with queen-worker dimorphism, workers cannot mate but retain functional ovaries1; because males arise from unfertilized haploid eggs, workers can potentially produce males. Worker-derived males are frequent in some species, but in others occur only in queenless colonies2,3. Workers are more related to their own sons (coefficient of 0.5) than to the queen's sons (their brothers; 0.25); they are also more related to nephews (0.375) than brothers if queens mate with one male, but if queens mate with more than two unrelated males a worker's mean rela tedness to nephews is less than to brothers3-5. In this case workers could benefit by 'worker policing'3,5: prevent-ing each other from producing males, perhaps by destroying worker-laid eggs or by aggression toward reproductive workers. Worker reproduction is rare in queenright colonies of species with multiply mated queens (such as honeybees6 and some yellowjacket wasps7), but is common in some monandrous species (bumblebees and stingless bees3). Here we describe experiments showing strong discrimination by honeybee (Apis mellifera) workers against worker-laid male eggs, supporting the worker-policing hypothesis. The honeybee was studied because queens mate with 10-20 males8, making worker policing seem likely as a cause for the rarity of worker-derived males (about one in a thousand males is worker-derived6).