Mate choice in polygynous species where males contribute little besides sperm to their offspring is of interest to evolutionary biologists, because the basis for such choice is often obscure and its adaptive significance, if any, controversial. I studied mate choice in the field crickets Gryllus veletis and G. pennsylvanicus (Orthoptera: Gryllidae) to determine the effects of male size, age, calling behavior and gregarine parasite levels on male mating success. I focused on the relation between parasites and sexual selection because recent work by Hamilton and Zuk suggests that the coevolution between host and pathogen provides a mechanism for genetic quality-based female choice and the evolution of sexually dimorphic characters. The number of females a male attracted was significantly correlated only with his age, and not with gregarine load, body size, or calling duration. Gregarine load was, however, negatively correlated with the number of spermatophores a male produced in 24-32 hours, and because females produce more eggs when multiply insemianted, males producing several spermatophores are expected to sire more offspring than males producing single spermatophores. Spermatophore production was unrelated to male size or age, but peaked in the laboratory in the early morning, coincident with peaks in mating and courtship song observed in the field. Comparison of field-collected males found with females to males found calling by themselves revealed that the paired males were significantly older and less parasitized than solitary males. Gregarines were common in both cricket species, with 30-70& of all individuals examined found to be infected. G. veletis males were more heavily infected than females; in G. pennsylvanicus the sexes were equally infected. The frequency of infected individuals fluctuated through the season. In crickets fed ad libitum, gregarine infection did not affect adult longevity, weight change or fecundity; food-deprived infected crickets lived fewer days and lost more weight than uninfected crickets. Development of nymphs was slower in crickets found to harbor gregarines at adult molt. These results suggest that female Gryllus may be making an adaptive mate choice, and that debilitating parasites may be important in sexual selection and more general aspects of cricket biology.