Lactobacillus murinus, a new species isolated from rat and mouse digestive tracts, is described. It belongs to the subgenus Thermobacterium although it is able to ferment pentoses. The electrophoretic mobilities of soluble proteins and lactic dehydrogenase, the guanine + cytosine content (43.7%), the L-lysine-D-Aspartic acid cell wall (b type, Kandler), the immunological relationship with other Lactobacillus species and also the aminoacid requirements show that it has to be assigned to a new species, Lactobacillus murinus. This species settles in the digestive tract of rodents in their first days of life and remains ther as a dominant flora throughtout the whole life of the animal.