Data published by R. Y. Stanier, N. J. Palleroni, M. Doudoroff and their colleagues on Pseudomonas have been analysed by numerical taxonomy. Records on 401 strains were used, representing 155 characters, mostly utilization of substrates as carbon-energy sources. Twenty-nine phenons were recognized, which included 394 strains: the remaining 7 remained unclustered. The results were in very good accord with the conclusions of these authors. Almost all phenons were well separated with very little overlap. Many of them corresponded to distinct species, and others corresponded to recognized biotypes. Some small groups may represent unnamed new species. Analyses by Gower's Coefficient showed five major groupings: A) the fluorescent pseudomonads; B) biochemically active species (Pseudomonas cepacia, P. pseudomallei and allies); C) moderately active free-living species (P. acidovorans, P. alcaligenes and allies); D) P. solanacearum and allies; and #) P. mallei P. diminuta does not appear to be clearly distinct from P. vesicularis, nor does P alcaligenes appear clearly distinct from P. pseudoalcaligenes. There may, however, be some difference between P. multivorans and P. cepacia. Analyses using the Pattern Coefficient differed mainly in the relationships shown by a few of the metabolically active species. Of the two coefficients, the Pattern Coefficient gave results that were in somewhat better agreement with evidence from nucleic acids, but it showed an unexpectedly close relationship between P. solanacearum and P. cepacia.