The systematics of the Lactobacillus population of the intestines of 88 different rats was studied; 80 rats had been fed on fermented oat-meal soup (Molin et al. 1992). One-hundred-twenty-two Lactobacillus strains from the intestinal mucosa were phenotypically classified together with twenty-eight reference strains of Lactobacillus and Leuconostoc, using 49 unit characters. Data were examined using Jaccard coefficient, and unweighted pair group algorithm with arithmetic averages. Two major and eleven minor clusters were defined at the 76% SJ-similarity level: Cluster 1 included thirty isolates which could not be identified further, but had resemblance to the type strains of L. jensenii, L. gasseri, L. crispatus, and to some extent to L. acidophilus. Cluster 12 including fifty-four intestinal isolates was identified as L. reuteri; and so was cluster 13 (five isolates). Isolates of the major clusters were found in all parts of the intestines. The genomic homogeneity of the L. reuteri isolates was scrutinized by endonuclease restriction analysis of the chromosomal DNA, and the isolates could be divided into six genomic strains.