Two important relationships in parasitoid evolutionary ecology are those between adult size and fitness and between host quality and sex ratio. Sexually differential size-fitness relationships underlie predicted sex-ratio relationships. Despite each relationship receiving considerable attention, they have seldom been studied simultaneously or using field data. Here we report the biology of Anoplius viaticus paganus Dahlbom, a little known parasitoid of spiders, using field and laboratory data. We found that larger foraging females were able to select larger host spiders from the field, thus identifying a relatively novel component of the size-fitness relationship. Larger offspring developed from larger hosts and, in agreement with the prediction of the host quality model of sex allocation, were generally female. Data on the size-fitness relationship for males are lacking and, in common with many prior studies, we could not evaluate sexually differential size-fitness relationships as an explanation for the observed sex-ratio patterns. Nonetheless, A. v. paganus exhibited one of the strongest relationships between host size and offspring sex ratio yet reported.