The m o s t probable n umber (MPN) o f hydrocarbon-degrading microorganisms found in the coastal waters a t Port Kelang, Port Dickson, Kuala Selangor, Malacca and Kuala Trengganu were 451, 513, 1163, 1388 and 14 8 c e lls/100 m l respectively. The results indicated that the population of hydrocarbon-degrading microorganisms in the Straits o f Malacca were 3 - 1 0 limes higher than that found in Kuala Trengganu (South China Sea). A significant level o f the hydrocarbon-degrading microorganisms was also detected in the sediment of the Kelang estuary. These microorganisms were active in oil degrading and may play an important role in degrading oily substances bro u g h t down from Kelang River. The studies o f the biodegradation of crude oil and selected paraffin hydrocarbons in the above coastal waters indicate that the hydrocarbon-degrading microorganisms present in Malaysian coastal waters have high a bility in utilizing hydrocarbons.