Many species of parasitic Hymenoptera inbreed regularly and do not suffer inbreeding depression. However, these inbreeding species could experience outbreeding depression due to the breakup of coadapted gene complexes. We measured the effects of outbreeding on three fitness components of the ectoparasitoid Nasonia vitripennis Walker, a chronic inbreeder. Experimental treatments varied the mating structure within and among populations: inbred (sib-matings within families), outbred within strains (nonsib-matings among families within strains), and outbred among strains, for 15 generations. Life span, fecundity, and sex ratio were measured in the parental generation and in four selected filial generations. We found no evidence of outbreeding depression for any of the fitness parameters tested. However, modest inbreeding depression was suggested as increased life span and fecundity when females were paired with nonsibling males from the same geographic strain, and by increased fecundity for interstrain crosses. Sex ratio did not respond to any of the levels of outbreeding. Consequently, we reject our hypothesis that N. vitripennis suffers outbreeding depression. However, heterosis within and among strains suggests that at least some local populations of N. vitripennis may suffer from modest inbreeding depression.