SUMMARY: Thirteen 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% guanine+cytosine (GC), and the DNA of the ten strains of P. diminuta from 66.3 to 67.3 moles%. One of the 13 strains examined had DNA of significantly lower GC content (62.2 moles%) and also differed in several phenotypic respects from both species; it is probably a monotypic representative of a third species, as yet unnamed.