The bacterial strain mA3 capable of utilizing 3-aminophenol as the sole source of carbon, energy and nitrogen for growth was isolated from an enrichment culture and identified as an Arthrobacter species. Utilization of 0.68 mg/ml 3-aminophenol by batch cultures of this organism was characterized by a specific growth rate (mu) of 0.18 h-1 and a yield coefficient (Y) of 0.60. In chemostat cultures of strain mA3 we determined a critical dilution rate (Dc) of 0.175 h-1 by continuous addition of mineral salt medium with 0.5 mg/ml 3-aminophenol. Evidence was obtained that the degradation of catechol by 3-aminophenol induced as well as non-induced cells follows the beta-ketoadipate pathway. The excess ammonium ions, originating from 3-aminophenol degradation and not needed for assimilation were released into the medium. Cells adapted to 3-aminophenol exhibited a high substrate specificity. Among different aromatic substances tested, only catechol and 3,4-dihydroxybenzoate could serve as a carbon source for growth. The importance of the meta position of the amino group for the first step of hydroxylation is discussed in connection with the substrate specificity of whole mA3 cells.