We determined the prevalence and intensity of blood parasites in breeding gray catbirds (Dumatella carolinensis) at Killbuck Wildlife Area in Wayne and Holmes Counties, Ohio (USA) from June through August 2000. Of 98 catbirds sampled, 40 (40.8%) had detectable infections of Haemoproteus beckeri. Overall prevalence of H. beckeri in this population is high relative to that reported in earlier blood parasite surveys of both breeding and migrant catbirds. Mean intensity of H. beckeri infection did not vary significantly between young and old birds or among sampling periods. We found no effect of age on prevalence or intensity of H. beckeri infection. Older birds were not more likely to be infected than younger birds, despite longer exposure to arthropod vectors. Prevalence varied significantly with season and was highest in June and lowest in August. This pattern also was observed in older birds sampled repeatedly. This seasonal variation may reflect both newly acquired infections and chronic infections relapsing in response to hormonal changes associated with breeding. Evidence of transmission was observed in the single hatching year bird that lacked detectable infection in early summer, but demonstrated a very high intensity infection in late summer. These observations provide supportive evidence that hematozoa infections are acquired on the breeding grounds during the first year of life and relapse during the breeding season in subsequent years.