SUMMARYThirteen strains previously assigned to the species Pseudomonas diminuta and P. vesiculare were subjected to detailed characterization. Ten of these strains could be identified as P. diminuta and two as P. vesiculare. These two species can be readily distinguished by differences in their nutritional spectra, their growth factor requirements and their pigmentation. The DNA of the two strains of P. vesiculare contained 65.8 moles yo guanine +cytosine (GC), and the DNA of the ten strains of P. diminuta: from 66.3 to 67.3 moles yo. One of the 13 strains examined had DNA of significantly lower GC content (62.2 moles yo) and also differed in several phenotypic respects from both species; it is probably a monotypic representative of a third species, as yet unnamed.All the strains examined share a series of distinctive properties, which justify their recognition as a special subgroup of aerobic pseudomonads. The defining characters of this subgroup include : monotrichous flagellation, with flagella of very short wavelength; a requirement for pantothenate, biotin and cyanocobalamin; a limited range of carbon sources; production of acid from primary alcohols by all strains that can utilize alcohols; inability to denitrify or to use nitrate as a nitrogen source; and accumulation of poly-P-hydroxybutyrate as an intracellular reserve.