A study was made of the epidemiology ofLeptospira interrogansserotypepomonainfection in an intensively housed herd of pigs. Diagnosis of infection was based on serological examinations. Blood samples were collected from groups of pigs at various intervals from birth until slaughter (six or seven months of age). Infection was confined to one of four buildings in which pigs were housed during this time. The movement of pigs between pens and the effluent drainage arrangements were factors responsible for infection becoming endemic in this building.