The biochemical characteristics of twenty‐five isolates of soft‐rot coliform bacteria from different countries are recorded. Because of the variability of the reactions, strains capable of producing potato black‐leg at both low and high temperatures, or at higher temperatures only, cannot be distinguished from one another with certainty by biochemical tests alone. They do, however, give an indication of potential pathogenic reactions. Thus the low‐temperature strains correspond withPecto‐bacterium carotovorumvar.atrosepticumand are distinguished from the high‐temperature isolates by their gas production, rapid fermentation of lactose and maltose, methyl red and Voges‐Proskauer reactions and failure to give a positive indole test.Because of their delayed lactose fermentation the chrysanthemum blight pathogen, the carnation slow wilt organism, the guayule bacterium and the organism causing leaf rot ofPhilodendronare classified as a new variety ofP. carotovorum(Jones) Waldee.The diagnosis of the coliform soft‐rot bacteria should be based on qualitative and quantitative biochemical reactions rather tha