We examined a population of northern leopard frogs to determine whether sex biases in investment in immunity, previously reported for this host species under controlled exposures to lung nematodes, is predictive of patterns of parasitism in nature. We examined Rhabdias ranae and Haematoloechus spp. infections in 74 breeding adult, 28 non-breeding adult, and 53 juvenile frogs. Contrary to our predictions, R. ranae prevalence and mean abundance were higher in breeding female frogs (prevalence: 39.4%, abundance: 3.05 ± 0.85) than on breeding males (prevalence: 26.0%, abundance: 1.17 ± 0.52), although no sex bias was observed among non-breeding adults or juvenile frogs. Female frogs also carried larger R. ranae worms, on average, than did males (females: 6,407.38 μm ± 153.80; males: 5,198 μm ± 131.09), regardless of age or breeding condition. We observed no sex-linked patterns of parasitism by Haematoloechus spp. worms in either adult or juvenile frogs. Alternative hypotheses, such as differences among sexes in the selection of thermal clines for hibernation, may explain the observed female bias in parasitism by nematode lungworms in nature and, thus, need to be considered.