Most theoretical work on brood sex ratio bias is based on life histories involving potential sibmating, where inseminated females colonize a habitat producing progeny that mate randomly among themselves. However, another type of life history can favour female biased broods; it involves motherson matings and is uniquely accessible to haplodiploids. Colonization is accomplished by immature stages (mating is postdispersal) and female bias is favoured at low colonization densities by the fact that, unlike isolated males, isolated females are not lost to the gene pool because they can mate with their parthenogenetically produced sons. We present a mathematical model of the life history including parameters describing colonization density, degree of aggregation, the penalty incurred when a female must wait to mate with her parthenogenetically produced son, and inbreeding. Low colonization density favours female bias as does increased aggregation; a high penalty associated with waiting for maturation of a son with which to mate means that some proportion of males among progeny will be favoured even at very low colonization densities. Life histories that fit the model are known in nematodes and mites.