Calodium hepaticum (syn. Capillaria hepatica), a nematode parasite commonly found in the liver of wild rodents, infects a wide variety of mammals, including humans. A retrospective study of black-tailed prairie dogs (Cynomys ludovicianus) at the Baltimore Zoo showed that 5 of 21 (24%) of the prairie dogs submitted for postmortem examination between 1981 and 2001 had hepatic capillariasis, with all the infections diagnosed during or after 1997. Affected livers contained multifocal granulomas containing numerous eggs and occasional adult nematodes. Asymptomatic wild rats in the zoo with a high prevalence of infection may have served as a reservoir for the disease. Wild rodent control is essential to minimize exposure of susceptible exhibition animals as well as humans to C. hepaticum.