Acinetobacter sp. strain ST-550 was isolated from fumus soil as an efficient indigo producer in the presence of organic solvent. The minimum inhibitory concentration of indole was 0.4 mg/ml for ST-550. ST-550 produced only a small amount of indigo (less than 0.01 mug/ml) when grown in the presence of indole at concentrations of 0.05 to 0.3 mg/ml without any organic solvent. However, ST-550 produced indigo effectively when grown in the presence of a large: volume of diphenylmethane and a high level of indole; optimized conditions were 3 ml of a medium containing 0.3 ml diphenylmethane and 2.7 mg indole. Under these conditions, ST-550 produced 0.88 mg indigo (292 mug/ml medium).