Field sex ratios of the solitary ectoparasitoid wasp Heterospilus prosopidis Viereck were examined. H. prosopidis mates panmictically in the field, and the size of their bruchid hosts vary. According to the theory that combines local mate competition and host quality effects, two major predictions for H. prosopidis sex ratios in the field can be made: more females should emerge from larger hosts, and 1:1 or slightly male-biased sex ratios should be observed in a natural population. Prosopis seeds infested with bruchids were collected in Hawaii, Arizona, and Texas in 1996, 1997, and 1998. Among nine collections, six did not deviate significantly from a 1:1 sex ratio, one showed a male-biased sex ratio, and two were female-biased. Although environmental factors other than host quality and mating structure may influence sex ratio temporally or spatially, these results generally support our predictions. none