A total of 48 strains representing the seven Pseudomonas stutzeri genomovars (DNA/DNA homology groups) were studied for cellular fatty acid composition, physiological characteristics and protein profiles. All strains were found to be homogeneous with respect to their fatty acid patterns. Numerical analysis of physiological properties demonstrated a considerable phenotypic heterogeneity within the genomovars. Characterization of the individual genomic groups on the basis of biochemical tests was not possible. In a numerical study of cellular protein patterns, two main clusters were obtained, one representing genomovars 1, 6 and 7 characterized by a high G + C content (mean value > 64 mol%), the other representing genomovars 2, 3, 4 and 5 with a low G + G content (< 64 mol%). The standardized cellular protein patterns have potential for differentiation of the genomic groupings within the species Ps. stutzeri as currently circumscribed.