This study examines the seasonal changes in a population of cosmocercid nematode parasites in a natural environment in an amphibian host of the Neotropical realm. A total of 232 individuals (83 females, 105 males, and 44 larvae) of Cosmocerca podicipinus were collected from 105 Pseudopaludicola boliviana from Corrientes, Argentina, between December 2002 and December 2003. The prevalence of infection was high during all the seasons (>80.0 %); the mean intensity decreased from summer to winter and then increased in spring. The sex ratios of C. podicipinus were significantly different (P < 0.05) from the expected 1.00:1.00 ratio in winter and in the entire study period. There were no significant differences in the mean length of female and male nematodes between seasons (P > 0.05). All adult nematodes collected (both males and females) were mature. The pattern of distribution of nematodes was aggregated in the entire study and in summer, autumn, and spring. The body size of the host was not related to the intensity of infection (P > 0.05). The seasonal occurrence of C. podicipinus in P. boliviana did not show a marked seasonality.