Four crude oils (Arabian light, Iran light, Oman, Libya) of different chemical composition were tested as to their biodegradability under mesophilic condition. Changes of the chemical composition of the crude oils by Xanthomonas campestris M12 were monitored using the gas-liquid chromatography. The ability of X. campestris M12 to use crude oil as a sole carbon source was not dependent on the composition of the n-saturate or aromatic fraction, but on that of the asphaltic fraction. The biodegradability of the Arabian light crude oil was found to be highly dependent on an incubation temperature. It was shown that addition of 0.1 % tween 80 to the incubation medium increased bacterial growth and biodegradability at 30℃ rather than that at 37℃.