In the pseudoscorpion, Cordylochernes scorpioides, males experience sexual selection in two disparate and well–defined habitats. Populations inhabit decaying trees for several generations before dispersing under the elytra of the harlequin beetle, Acrocinus longimanus. Males compete to monopolize beetle abdomens as strategic sites for inseminating dispersing females. Using single–locus minisatellite DNA profiling to assign paternity for the offspring of dispersing females, we found a strong, positive correlation between male size and reproductive success in the beetle environment. However, this intense selection is undermined by polyandry and the ability of females to store sperm and produce mixed–paternity broods. Although beetle–riding males achieved fertilizations with 70 per cent of the females, paternity could not be assigned for 57 per cent of the offspring. It is likely that many of these offspring were the products of within–tree inseminations since, in a sample of females intercepted in the act of boarding beetles, most (86 per cent) carried sperm from pre–dispersal matings within trees. Polyandry and sperm storage may therefore enable smaller males, unable to monopolize beetle mating territories, to circumvent the bottleneck of dispersal–generated sexual selection and thereby transmit their genes to future tree populations. Sperm stored within females can thus provide the kind of resistant life–history stage shown by recent modelling to be critical for the maintenance of genetic variation by temporally fluctuating selection.