Fatty acid profiles of 202 coryneform and nocardioform bacteria were recorded by gas chromatography. Strains were grouped according to their profiles using mean linkage cluster analysis and similarity measures based on the correlation coefficient, the angular separation between vectors in a multidimensional space and the degree of overlap between superimposed traces. Comparisons using both real and hypothetical data showed the last of these measures to be the most effective. Strains were divided into two major groups, depending on whether they contained predominantly straight chain or iso-and anteiso-branched acids. The first group was divided into two subgroups according to the relative proportions of the characteristic acids present ; one subgroup had six clusters containing the rhodococci, nocardiae, mycobacteria and caseobacters, and the other had two containing the xanthobacters and true corynebacteria. The second group was divided into one subgroup containing strains of Arthrobacter simplex, Arthrobacter tumescens and Arthrobacter duodecadis, and one having three clusters. One cluster from this latter subgroup contained cellulomonads, one contained brevibacteria and curtobacteria and one contained arthrobacters, oerskoviae and kurthiae. Identification to generic level by fatty acid composition alone may not be feasible, but fatty acid analysis coupled with morphological examination may be sufficient to identify Corynebacterium, Arthrobacter, Cellulomonas, Oerskouia, Brevibacterium, Caseobacter, Kurthia and the A. simplexltumescens taxon. Distinction is not easy between Curtobacterium, Microbacterium and the diaminobutyric acid-containing coryneforms and between Rhodococcus, Mycobacterium and Nocardia.