The olfactory and contact behavioral responses of clover root borer, Hylastinus obscurus (Marsham) (Coleoptera: Curculionidae), against fatty acid components present in 9-mo-old red clover, Trifolium pratense L., roots were investigated by using an automated behavioral observation system. From GC-MS analysis of dichloromethane extracts of T. pratense roots, of 15 compounds identified in total, four long-chain free fatty acids lauric, palmitic, oleic, and stearic acids were found to be main components in the extracts. In a four-arm olfactometer the clover root extract significantly attracted both male and female H. obscurus. When solutions of the four individual synthetic fatty acids and their blend at the ratio found in the root extract (10 mu g/ml) were assayed with H. obscurus, lauric, palmitic, and oleic acid elicited an attractant behavioral response from females, whereas all substances tested did not elicit a response from males. In contact bioassays, wood dummies coated with root extract, the four fatty acids, or their respective blend of the same composition found in the root extract at 100 mu g/ml, were significantly preferred by female H. obscurus. In contrast, males showed preference only for dummies treated with root extract, or palmitic or oleic acid. The behavioral evidences suggest that long-chain free fatty acids present in 9-mo-old red clover roots may play a role as close-range olfactory, tactile cues, or both in the host-finding process of H. obscurus.