Dispersal influences a variety of ecological and evolutionary dynamics including metapopulation persistence and local adaptation. Sex-biased dispersal evolves when the costs and benefits associated with dispersal differ between the sexes. These costs and benefits may be fixed, resulting in a consistent pattern of sex-biased dispersal within species whereby one sex always disperses more and/or further than the other. Alternatively, the costs and benefits may vary depending on the intensity of competition experienced by the two sexes. In this case, the direction of the sex bias may be plastic and depend on the sex ratio of the population. In the current study, we asked whether a semiaquatic, flight capable insect (Notonecta undulata) exhibits sex-biased dispersal and whether the strength of intrasexual competition experienced by males and females determines the direction of the sex bias. We conducted a mesocosm experiment in which we manipulated the population sex ratio and measured the probability of dispersal for males and females. We found that while both sexes dispersed, male dispersal rates were higher, and this pattern was independent of sex ratio. This suggests that fixed sex-specific dispersal costs and/or benefits are likely to be more important determinants of sex-biased dispersal in notonectids than population sex ratio.