The aggregation pheromone (a mixture of three components) of Riptortus clavatus (Heteroptera : Pentatomidae) attracted nymphs and both sexes of adults of Piezodorus hybneri (Heteroptera : Alydidae). Large numbers of P hybneri were caught in a pheromone trap placed in a soybean field, compared with very few in a trap placed in grassland 120 m away from the soybean field, but equal numbers of R. clavatus were caught in both traps. Among three components of the pheromone. (E) -2-hexenyl (E) -2-hexanoate (E2HE2H) showed the attractiveness to P. hybneri. and its activity was almost equal to that of the aggregation pheromone. Since E2HE2H is the major component (ca. 71%) of the pheromone and the other components did not show attractiveness, we consider E2HE2H to be the component that attracted P hybneri. P hybneri seems to use this component of the pheromone of R. clavatus, which has a similar host range. as a kairomone to search for food plants.