The fatty acid, menaquinone and polar lipid composition of Arthrobacter simplex, A. tumescens, "Brevibacterium lipolyticum", Nocardioides albus and N. luteus were examined. The results of the present and earlier studies indicate A. simplex and A. tumescens should be removed from the genus Arthrobacter. On the basis of lipid composition A. simplex, "B. lipolyticum", N. albus and N. luteus are closely related to each other but are quite distinct from all other coryneforms and actinomycetes examined to date. Arthrobacter tumescens possess a very characteristic fatty acid and polar lipid composition and on that basis can be clearly distinguished from A. simplex and related species and possibly warrants a new taxon.