Hymenoptera are characterized by a haplodiploid sex determination system where females are diploid and males haploid. Because females can decide whether they fertilize their eggs they have control over the sex of their offspring. Females are thus expected to lay haploid and diploid eggs in a ratio that maximizes their fitness. A surprising finding of recent studies on the primary sex ratio of ants has been that queens lay male eggs outside the time of production of sexuals, a period during which all haploid eggs are usually destroyed by workers. It has been suggested that the production of a high proportion of haploid eggs by queens might be a mechanism to achieve higher reproductive success when the colony loses all its queens and rears new sexuals from the brood present within the colony. Here it is shown that, as predicted by this hypothesis, the proportion of haploid eggs laid by Linepithema humile queens is higher in colonies containing several queens than in single-queen colonies. Because L. humile sexuals mate in the nest, the females that produce a higher proportion of haploid eggs should have higher fitness when the colony sex ratio of the brood is female-biased and sons of several queens compete to fertilize the new queens produced. (C) 1996 The Association for the Study of Animal Behaviour