We use allozyme polymorphism to estimate foundress numbers and inbreeding in a Swedish population of the parasitoid wasp Nasonia vitripennis and compare observed sex ratios with the predictions of local mate competition theory. Foundress numbers per host patch estimated from electrophoretic data range from one to nine, with an arithmetic mean of 2.3 and a harmonic mean of 1.49. The inbreeding coefficient calculated directly from the genetic data (F = 0.312) was in agreement with that estimated from the foundress number distribution (F = 0.336). The proportion of males per patch increased as predicted by the estimated foundress numbers and the level of inbreeding, particularly when minimum numbers of males per brood and synchrony of emergence are taken into account. Superparasitism was more frequent than expected from experimental studies. The mating structure of this population implies that it is probably under selection for both female-biased sex ratios and facultative sex ratio behaviour because of variable opportunities for outbreeding.