This study examines the sex ratio response of the parasitoid wasp Muscidifurax raptor to conspecific and confamilial females in relation to two groups of functional sex ratio models, local mate competition and host quality models. In some but not all experiments, M. raptor females produced a greater proportion of sons in the presence of a conspecific female than when alone, and this sex ratio effect carried over for a day after the females were isolated from each other M. raptor females also produced a greater proportion of sons in the presence of a female of the confamilial parasitoid Spalangia cameroni than when alone (although only on the second day of exposure to S. cameroni, not on the first). M. raptor's sex ratio increase in the presence of conspecifics is consistent with local mate competition models but not with host quality models because the presence of a conspecific female did not cause there to be more, and thus potentially smaller, offspring developing per host. In contrast, the presence of a S. cameroni female did cause there to be more offspring developing per host than when a M. raptor female was alone; thus M. raptor's sex ratio increase in the presence of S. cameroni may be explained by host quality models. An alternative explanation for the sex ratio increase in response to confamilials is that only a sex ratio response to conspecifics may be adaptive, due to local mate competition; but M. raptor females may be unable to distinguish between conspecific and S. cameroni females.