A toluene-degrading bacterial strain isolated from bioreactors was identified as a member of the genus Rhodococcus on the basis of the following characteristics: meso-diaminopimelic acid, arabinose, galactose, and glucose are the diagnostic cellular sugars; the mycolic acids contain 33 to 36 carbon atoms; and the formation of a branching mycelium is followed by marked fragmentation of the mycelium into irregular rod-shaped to coccoid units. DNA-DNA hybridization analyses performed with type strains of Rhodococcus species showed that this strain is less than 70% related to other species that have similar phenotypic characteristics. On the basis of these findings, we propose that this strain should be described as a new species, Rhodococcus zopfii, in honor of Wilhelm Friedrich Zopf. The type strain is strain T1.