Female promiscuity has broad implications for individual behaviour, population genetics and even speciation. In the field cricket Gryllus bimaculatus, females will mate with almost any male presented to them, despite receiving no recorded direct benefits. Previous studies have shown that female crickets can benefit from polyandry through increased hatching success of their eggs. There is evidence that this effect is driven by the potential of polyandrous females to avoid fertilizing eggs with sperm from genetically incompatible males. We provide direct evidence supporting the hypothesis that polyandry is a mechanism to avoid genetic incompatibilities resulting from inbreeding. Using microsatellite markers we examined patterns of paternity in an experiment where each female mated with both a related and an unrelated male in either order. Overall, unrelated males were more successful in gaining paternity than were related males, but this effect was driven by a much greater success of unrelated males when they were the first to mate.