Following the introduction of the DNA hybridization technique several genotypes have been separated from the older phenotypically described Aeromonas species. Work has been undertaken on some arbutin-negative psychrotrophic Aeromonas strains. These were differentiated into three genotypically and phenotypically identifiable groups. One group (I) is genetically related to A. sobria (type 208). The two other groups (II and J) can be separated genotypically and phenotypically from the A. sobria, A. veronii and A. hydrophila genotypes. Studies on ADH-negative anaerogenic surface water aeromonads showed them to be genotypically more distant from A. punctata type 239 than from type 545.