none Sex ratios of offspring in forty-one broods of Asobara persimilis (an hymenopteran parasitoid of Drosophila larvae so that local mate competition would be expected to occur amongst their offspring. The parental population had a sex ratio of 1:1 (87F:102M), the offspring sex ratio was 787F:296M (2.66:1) highly female biased and not significantly different from 3:1 (P>0.05). The sex ratio was approximately constant regardless of the total brood size which ranged from 5 to 100 (mean 26.2). Females were therefore able to adjust their offspring sex ratios in the predicted direction and the number of males in each brood was determined binomially.