The assimilation of carbon substrates by 103 strains of Aeromonas of different origin identified by conventional methods was studied by means of a standardized micromethod containing 147 tests (API system). Six distinct groups could be recognized and the discriminating substrates were determined. 3 species of Aeromonas can be identified by means of conventional method: A. hydrophila, A. sobria and A. caviae. The method has a number of drawbacks: Some media are unreliable, others are difficult to read, strict preservation conditions are essential. The proposed micromethod for carbon substrate assimilation allows, in most cases, a simple separation of the 3 motile Aeromonas species.