Seventy-five per cent of 845 Norway rats examined in the Baltimore Zoo for Capillaria hepatica were infected. Nearly all adult rats and 65% of juveniles were infected. Only 8% of 299 infected rats were heavily infected. The prevalence and intensity of infection increased with the size of the host. There were no seasonal differences in infection rates among adults, but juveniles collected during spring had higher infection rates than those collected during winter. Prevalence of C. hepatica infection varied from one place to another. No correlation between infection rate, vegetative cover, soil type, monthly rainfall, mean daily temperature, or food habits of rats was found. The dynamics of rat populations are perhaps the most important factors in the maintenance and dynamics of C. hepatica infections. Rapid population turnover contributes to the rapid release of a great number of eggs into the environment and high recruitment rates provide sufficient numbers of susceptible hosts for the parasite to complete its cycle.