Phenotypic characteristics were used to identify 189 Aeromonas strains isolated from human feces. One hundred forty-two of these strains were placed in 11 DNA hybridization groups, and the genetic and phenotypic data were compared. According to the criteria of Popoff, 66% of the strains were identified as Aeromonas caviae, 18% were identified as A. sobria, and 16% were identified as A. hydrophila. Some biochemical characteristics differed from the criteria of Popoff; 19 of 40 (48%) of tested strains were encapsulated, 42 of 124 (34%) of A. caviae strains were nonmotile, and all A. sobria strains were resistant to KCN. Gas production from D-glucose was temperature dependent; 11 of 64 (17%) A. hydrophila and A. sobria strains produced gas only at 22 degrees C. Of 142 Aeromonas strains, 57% belonged to hybridization group 4, 25% belonged to group 8, 11% belonged to group 1, 4% belonged to group 5A, 2% belonged to group 3, and 1% belonged to group 2. Of 26 strains phenotypically identified as A. hydrophila, 8 (31%) were in hybridization group 8, which contains strains of the new species A. veronii. It therefore appears that our ability to identify Aeromonas strains phenotypically is not sufficiently specific. Either additional definitive biochemical markers must be found or phenotypic identification, at least for some Aeromonas groups, must be regarded as only presumptive.