The mating behavior of the male Gastrophysa atrocyanea, that is, extrusion of the aedeagus, was elicited by the cuticular hydrocarbons on the surface of the female body. Males attempted to mate with a dummy treated with the female equivalent of the female elytra extracts. After fractionation of the elytra extracts by Florisil column chromatography and reverse phase high performance liquid chromatography, two fractions, consisting mainly of methylheptacosanes and methylnonacosanes were found to elicit the male mating behavior. These compounds, synthetic 9-methylheptacosane, 11-methylheptacosane, 9-methylnonacosane and 11-methylnonacosane, were contained in active fractions and showed high pheromonal activity. Therefore, these monomethylalkanes were considered to be contact sex pheromones of this beetle. However, synthetic 13-methylheptacosane, 13-methylnonacosane and 15-methylnonacosane, positional isomers of active monomethylalkanes, also contained in the active fractions, did not have the activity. A mixture of these seven monomethylalkanes at approximately natural concentrations also had enough activity to elicit the male mating behavior. Furthermore, the results of a bioassay using some related structural monomethylalkanes revealed that the length of the carbon backbone and the position of the methyl branch greatly influenced the pheromonal activity of G. atrocyanea.