Parasite host specificity has important implications for species diversity estimates, food web dynamics, and host shifts. “White grub” is the metacercaria stage of a fluke (Posthodiplostomum minimum) that occurs in many fish species, but no attempt has been made to quantify variation in host use by this worm. Here we used 2 approaches to evaluate host specificity within the strain that infects centrarchids (P. minimum centrarchi). First, we measured parasite loads in 2 centrarchid hosts, bluegill (Lepomis macrochirus) and white crappie (Pomoxis annularis), from Spring Lake in McDonough County, Illinois. We found that infection levels differed significantly between these hosts. Prevalence in bluegill was 100% and the median intensity was 940 metacercariae, but only 57% of white crappie were infected (median intensity = 4). Site specificity of white grub also differed significantly between the 2 hosts. In bluegills, kidneys were most heavily infected, whereas in white crappies, livers harbored the most worms. We also performed a literature survey of P. minimum prevalence estimates from 14 centrarchid species from other localities. We calculated the mean white grub prevalence for each host species and used this to calculate STD*, a quantitative index of host specificity. STD* was 1.33, significantly closer to the value for a specialist (STD* = 1.00) than a generalist (STD* = 2.00). This reflects the fact that P. minimum prevalence is higher in Lepomis species than it is in centrarchids outside this genus. These data show that P. minimum centrarchi specializes on Lepomis species, but the causes of this specialization are unknown. This worm may be a single species that differs in host use due to ecological or physiological host differences, or it may be a complex of species that vary in host use for similar reasons. Genetic data are required to evaluate these possibilities.