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The antitermitic activities of leaf oils and their constituents, taken from three clones of Eucalyptus camaldulensis Dehnh. in Thailand, against Coptotermes formosanus Shiraki were investigated in contact and noncontact tests. The termiticidal mechanism was also examined. Antitermitic tests demonstrated that E. camaldulensis leaf oils were both contact toxicants and fumigants to C. formosanus with LC 50 values ranging between 12.68 and 17.50 mg/ g by the contact method, and between 12.65 and 17.50 mg/ petri dish (100 cm 3 ) by the noncontact method. p-Cymene and g-terpinene were primarily responsible for the contact toxicity and 1,8-cineole was responsible for fumigation. From the investigation of termiticidal mechanism, E. camaldulensis leaf oils exhibited the inhibition of acetylcholinesterase activity and showed the common symptoms of a neurotoxic mode of action against C. formosanus.