Four hundred forty-three Norway rats (Rattus norvegicus) were examined to determine their role in the transmission and maintenance of Trichinella spiralis on a pig farm. Rats, classified by sex and weight, were examined for trichinellosis by peptic digestion of muscle samples. Over a 25-mo period, 188 (42.4%) rats were found to be infected with T. spiralis. The mean intensity of infection was 293.2 larvae per gram (LPG) of muscle; 65 (34.6%) infected rats had intensities of infection greater than 100 LPG. Even in the absence of a known source of infected meat (garbage containing meat scraps or dead animals), the rat population maintained the infection, probably through cannibalism. Population reduction was an effective method for reducing the prevalence of infection within the rat population. Therefore, to reduce the likelihood of transmission of T. spiralis between rats and swine, it is essential that rat populations in a farmyard environment be controlled.