1. Differential dispersal of males and females from a population is predicted to result in biased sex-allocation decisions, even in the absence of sibmating. 2. Mated Bracon hebetor Say (Hymenoptera: Braconidae) females produce distinctly female-biased sex ratios (approximately 30% male), yet sibmating is not a feature of the mating biology of this species. Therefore the dispersal behaviour of male and female B. hebetor from caged subpopulations was examined. 3. A higher proportion of females than males dispersed from the caged subpopulations. Furthermore, females dispersed earlier than males. This suggests that the level of competition for mates experienced by males is higher than the level of competition for hosts experienced by sisters. 4. Roughly half of the dispersing females left after they had mated. Females generally mate once in their lifetimes, suggesting that competition between brothers for mates may be high. none