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Peromyscus leucopus populations exhibit unstable population dynamics. Mathematical models predict instability with chronic parasite infections that reduce host fecundity when the parasite distribution within the host population is close to random. We examined the role the nematode Pterygodermatites peromysci may play in influencing the dynamics of these mice. There were seven gastrointestinal worms infecting mice. Pterygodermatites peromysci was the most prevalent and varied seasonally from 12.3% in November to 36.0% in July. Prevalence was higher in adults (30.8%) than juveniles (4.6%) and there were no statistical differences in prevalence or intensity between the sexes. Overall the distribution was random; the relationship between log variance and log mean of P. peromysci intensity from 17 sites was not significantly different from unity. There were significant relationships between infection and breeding condition, suggesting parasites could be the cause of reduced female breeding. A generalized linear model found the likelihood of P. peromysci infection in adults increased with body mass, the presence of other helminths, and when hosts were in breeding condition. Likewise, the intensity of infection was positively related to co-infections and body mass. Pterygodermatites peromysci infection was associated with the presence of the oxyurid nematode Syphacia peromysci but co-infection was lower in females than males. Amongst females, co-infection was greater when breeding, particularly during lactation. The P. peromysci age-intensity relationship increased with age and rose to an asymptote as expected for a parasite with constant mortality and no acquired immunity. Overall, P. peromysci had a random distribution and was associated with reduced breeding; we discuss how these destabilizing processes may influence the dynamics of P. leucopus.