Microbial oxidation of aliphatic primary alcohols was done on the interface between a hydrophilic carrier and a hydrophobic organic solvent (interface bioreactor). Issatchenkia scutulata var, scutulata IFO 10070 produced octanoic and decanoic acids in high yields on the interface between the agar or agar-coated filter pad and the decane layer. This solid-liquid interfacial oxidation system was superior to an emulsion system.