A filamentous, segmented bacterium was observed in the small intestine of the SSC:AH stock of mice from the Statens Seruminstitut (Denmark) animal colony but was absent in golden hamsters and guinea pigs from the same colony. The bacterium is attached to the epithelial cell by a special segment (holdfast) and causes specific changes in the epithelial cell at the site of attachment. Once attached the bacterium appears to undergo a complex life cycle which involves the development of a long filament divided into a number of segments within which holdfasts or spores are formed. This organism is morphologically identical to a bacterium found in mice and rats in the USA, but this is the first report of such an infection in Europe.