Seasonality has strong effects on natural systems by influencing resource availability, thereby interfering in plant–herbivore, prey–predator, and host–parasite interactions. We compared the seasonal structure of the helminth community of rufous-bellied thrushes (Turdus rufiventris), assessed its correlation with environmental variables, and interpreted temporal patterns of parasite abundance in relation to their life cycles and likely changes in the availability of intermediate hosts and vectors. Fifteen helminth species were found in a sample of 151 thrushes collected on a seasonal basis over 3 yr. Infracommunity structure was affected by season and year. The ordination of component communities grouped fall and winter samples within a gradient of similarity that was correlated with average relative air humidity (RH) and average ambient temperature. RH (alone or in combination with temperature, rainfall, or both) was also found to be a good predictor of the abundance of 5 helminth species.