The ability of a Rhodococcus aetherovorans strain, BCP1, to grow on butane and to degrade chloroform in the 0-633 mu M range (0-75.5 mg l(-1)) via aerobic cometabolism was investigated by means of resting-cell assays. BCP1 degraded chloroform with a complete mineralization of the organic Cl. The resulting butane and chloroform maximum specific degradation rates were equal to 118 and 22 mu mol mg(protein)(-1)day(-1), respectively. Butane inhibition on chloroform degradation was satisfactorily interpreted by means of a model of competitive inhibition, with an inhibition constant equal to 38% of the estimated butane half-saturation constant, whereas chloroform (at 11 mu M) did not inhibit butane utilization. Acetylene (1,720 mu M) induced an almost complete inactivation of the degradation of both butane and chloroform, indicating that the studied cometabolic process is mediated by a monooxygenase enzyme. BCP1 proved capable of degrading vinyl chloride and 1,1,2-trichloroethane, but not 1,2-trans-dichloroethylene. BCP1 could grow on the intermediates of the most common butane metabolic pathways and on the aliphatic hydrocarbons from ethane to n-heptane. After growth on n-hexane, it was able to deplete chloroform (13 mu M) with a degradation rate higher than that obtained, at the same chloroform concentration, after growth on butane.