A phenanthrene-degrading bacterium that assimilated a wide range of organic compounds was isolated from a soil sample and identified as Alcaligenes faecalis strain AFK2. The strain degraded phenanthrene through protocatechuate, but did not utilize naphthalene. The phenanthrene-degrading phenotype (Phn+) of AFK2 disappeared after 20 successive subcultures in a mineral salts medium containing o-phthalate or after subculture in nutrient broth containing mitomycin C. The results suggested that the Phn+ phenotype of this strain might be encoded by extrachromosomal genes.