The egg-releasing mechanism and transmission ecology of Capillaria hepatica among Norway rat populations of the Baltimore Zoo were studied from 1972 to 1974. Nearly all adult rats were infected, while 65% of juveniles had infections. The mean egg count per liver was calculated to be 457,783 (N = 39 livers) and ranged from 11,270 to 1,400,000 eggs per liver. Data from the present study suggest that cannibalism serves as a primary egg-releasing mechanism and is a source of infection within the burrows. Increased infection rates among juveniles in spring support the hypothesis of maintenance of C. hepatica infections within the burrow system through cannibalism. Predation was responsible for scattered foci of infection throughout the study area and considered as a secondary source of infection. Decomposition was an important egg-releasing mechanism in secondary foci and in the warmer season when insects were active. However, of 849 carrion-associated insects and soil invertebrates collected from around decomposing rats, eggs of C. hepatica were found in only two species of beetles. This suggests a minor role for insects and soil invertebrates as egg disseminators.