The addition of lactose to the diet of rats enabled the detection of Lactobacillus bifidus in the feces of the animals several days after the addition. In some cases, the number of L. bifidus predominated over that of others. Since L. bifidus synthesize thiamine from its moieties, a large amount of the vitamin observed in the cecum of the rats on the lactose diet supplemented with pyrimidine- and thiazole-moiety seems to be synthesized by the organism.