Urban brown rats (Rattus norvegicus) carry microbial human pathogens but their role as reservoir hosts for helminths of public health importance is less well known. In this study, 42 brown rats trapped on Merseyside were subject to thorough combined helminthological and pathohistological post-mortem examination. Eggs of the rodent-borne zoonotic nematode Calodium hepaticum were initially detected in histological sections of the livers of 9.5% of rats, but overall diagnostic sensitivity increased to 16.6% when entire liver tissue was disrupted and the resulting filtrates were examined for released eggs. In their rat host, mainly trapped inside the dockland, infections with C. hepaticum were associated with a chronic multifocal pyogranulomatous hepatitis with intralesional eggs and peripheral fibrosis. Mean intensity of hepatic C. hepaticum egg infections was 1041 eggs. This is the first report of C. hepaticum in an urban brown rat population in the UK and provides original data for liver egg burdens in this abundant commensal rodent. The zoonotic cestode Rodentolepis nana had a prevalence of infection of 14.3%. Rodent-specific, non-zoonotic helminths found were the spiruroid Mastophorus muris (16.0%) in the stomach, the trichuroid Trichosomoides crassicauda in the urinary bladder (31.0%); the ascarid Heterakis spumosa was the commonest helminth of the large intestine (76.2%). Many millions of brown rats inhabit cities and rural areas of the UK, and the infective stages of the zoonotic worm species, particularly C. hepaticum, are likely to be widely distributed in the environment presenting a threat to public health.