Of 38 strains of Propionibacterium avidum, most fell into one of two groups. One group (20 strains) had LL-diaminopimelic acid as the diaminoacid of peptidoglycan, did not ferment inositol, and reacted with serum to strain VPI 0576; the other group (11 strains) had DL-diaminopimelic acid as the peptidoglycan diaminoacid, fermented inositol, and reacted with serum to strain VPI 0670. DNA sequence similarity studies showed that, while overall intergroup similarity was about 80%, within each group the sequence similarities were over 90%. Seven strains were anomalous and did not fit exactly into either group. The results show that the isomer of diaminopimelic acid in peptidoglycan may differ in strains of a single species all of which show at least 80% DNA sequence similarity to each other.