A phenanthrene-degrading Mycobacterium sp. strain 6PY1 was grown in an aqueous/ organic biphasic culture system with phenanthrene as sole carbon source. Its capacity of degradation was studied during sequential inoculum enrichments, reaching complete phenanthrene degradation at a maximim rate of 7 mg l(-1) h(-1). Water-oil emulsions and biofilm formation were observed in biphasic cultures after four successive enrichments. The factors influencing interfacial area in the emulsions were: the initial phenanthrene concentration, the initial inoculum size, and the silicone oil volume fraction. The results showed that the interfacial area was mainly dependent on the silicone oil/mineral salts medium ratio and the inoculum size.