Axenic mice and rats fed different diets were associated only with two strains of Clostridium. The first one identified as Clostridium perenne was isolated from the fecal dominant flora of an adult rat. The second one belonging to the group I of Clostridium was isolated from the dominant flora of the feces from a piglet. Both strains were capable of becoming established in the digestive tract of animals fed dietary regimens called "permissive", while they did not become established in animals fed "nonpermissive" diets. However, when these bacterial strains had become established in animals fed a permissive diet, they persisted in the dominant flora even if the permissive diet was replaced by the nonpermissive one. This phenomenon was called remanent effect of the diet. It disappeared after 103 days for C. perenne, but not for the other Clostridium strain. This effect was not due to a selection of a genetic variant from the original strain or to a durable change in some characteristics of the host-animal associated with the bacterial strain.