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Reference strains (2) and 29 isolates of Aeromonas spp. from clinical material and environmental specimens were characterised in traditional biochemical tests, and in pyrolysis mass spectrometry, which gives data reflecting whole-cell composition. Numerical taxonomic analyses of the data sets were compared with conventional identification at species level, and pathogenic potential, as inferred from the origin of the isolates. Clustering with conventional test reaction patterns showed, for each of the species represented, a clearly defined core group of 'typical' isolates, surrounded by a halo of aberrant strains. One further cluster comprised strains intermediate between A. caviae and A. hydrophila, and one strain was grossly atypical in both analyses. Clustering from pyrolysis data corresponded less well with species identification. Broadly, the biochemical division between core and halo strains was supported in pyrolysis for A. caviae and A. sobria, but the main group of A. hydrophila in pyrolysis comprised strains clustering in the core and halo groups of this species, and three strains intermediate between A. hydrophila and A. caviae in biochemical tests. Two further pyrolysis clusters comprised core and halo strains of A. hydrophila. However, pyrolysis clustering correlated well with inferred pathogenicity, showing four clusters of probable pathogens, six clusters of probable non-pathogens, and one two member cluster of doubtful status. Most strains that clustered in the species haloes, or in species-intermediate groups in biochemical tests, were non-human isolates, or were isolated in the absence of symptomatic infection. The correlation of inferred pathogenicity with biochemical clustering was poorer than that with pyrolysis clustering.