The chromosomal DNA was isolated and purified from 17 strains of Pseudomonas paucimobilis, and from the type or reference strains of Flavobacterium capsulatum, F. devorans, F. multivorum, 'Chromobacterium lividum', Xanthomonas campestris and seven species of Pseudomonas. The DNA base compositions (mol% G + C) of P. paucimobilis strains were between 62.2 and 68.6%, and typical strains had a mean value of 65.3 +/- 0.4 mol%, determined from thermal denaturation temperature. DNA-DNA molecular hybridization with 3H-labelled probe DNA from NCTC 11030 P. paucimobilis (the type strain) indicated that the species comprised a core of 13 closely related strains (74 to 96%), which included F. devorans NCIB 8195 (= ATCC 10829). Four P. paucimobilis strains displayed lower levels of hybridization (less than or equal to 38%). The hybridization results showed that P. paucimobilis was not closely related to allied yellow-pigmented bacteria or to other reference pseudomonads. The electrophoretic protein patterns of representative strains were analysed by computer-assisted techniques and similarity coefficients were calculated. A high degree of congruence was obtained with the DNA hybridization data, and the protein analyses indicated that the four atypical P. paucimobilis strains were heterogeneous and not a single group within the species. The generic affinities of P. paucimobilis, F. capsulatum, 'P. azotocolligans' and P. echinoides are uncertain, but available chemotaxonomic data indicate these species could provide the basis for a new genus.