A total of 224 strains of the genus Pseudomonas sensu lato were studied by 99 carbon source utilization tests using BioMerieux Biotype-100 strips and 96 substrate oxidation tests using the Biolog GN Micro Plate system. Biotype-100 and Biolog data were computer analyzed using programs of the Taxotron package (Institut Pasteur) and the Bionum software (Biolog), respectively. Unweighted pair group method of averages (UPGMA) yielded 34 Biotype phenons and 36 Biolog phenons. A total of 18 phenons had identical composition in both systems. Six phenons were almost identical in structure whereas some phenons in one system were lumped in the other system. Some species (represented by at least three strains) constituted homogeneous phenons in both Biotype-100 and Biolog systems: Pseudomonas aeruginosa, P. tolaasii, P. mendocina, P. cichorii, P. viridif lava, P. fragi, P. stutzeri, P. agarici, P. alcaligenes, and P. pseudoalcaligenes. None of the two systems could discriminate P. fuscovaginae from P. asplenii, and P caricapapayae from P corona f aciens. P. corrugata was a tight subphenon in both systems. Strains labeled P. f luorescens were found in six phenons with either Biotype-100 or Biolog systems. Strains of P. putida were found in three (Biolog) or four (Biotype) phenons. Strains of P marginalis were distributed over five phenons in both systems. The use of Biotype-100 strips for Pseudomonas characterization was also validated by comparison with published carbon source utilization data.