The biochemical characteristics of 55 strains of Aeromonas spp. and 16 P. shigelloides of human faecal origin were examined. Results of tests for the production of oxidase, acid from xylose, dulcitol, adonitol, mannitol, or inositol; greenish pigmentation on nutrient agar, and growth in 6.5% NaCl broth were found to be specific and reproducible. However, other tests such as haemolysis on blood agar (BA), production of DNase, amylase, ornithine or lysine decarboxylase; citrate utilization and the methyl red, Voges-Proskauer (MRVP) varied with different strains. On the basis of our results, we propose a simple scheme for the preliminary identification of Aeromonas and Plesiomonas isolates in laboratories with limited resources. We hope that other laboratories will evaluate it to determine its validity.