In gregarious animals, group size correlates negatively with infection levels by some kinds of parasites and positively with infection by others. Conflicting selection pressures can be exerted simultaneously on a host species by different parasite species. Among stationary, mixed-species shoals of juvenile threespine sticklebacks, Gasterosteus aculeatus, and blackspotted sticklebacks, Gasterosteus wheatlandi, shoal size correlated differently with levels of infection by two species of ectoparasites. Stickleback shoal size correlated positively with infection levels by the copepod Thersitina gasterostei, which is transmitted among fish by short-lived planktonic larvae. In contrast, infection levels by the highly mobile crustacean parasite Argulus funduli did not decrease as shoal size increased, as predicted from an earlier laboratory experiment. The species composition of the different stickleback shoals also had an influence on some aspects of infection by these two parasite species. The contrasting mode of transmission of the two parasites results in one parasite species having a higher transmission rate among fish within large shoals, whereas the success of the other parasite species is independent of fish shoal size. The two ectoparasites may thus exert different selection pressures on stickleback shoal sizes.