Western boxelder bugs, Boisea rubrolineata (Barber), form large aggregations on pistillate boxelder, Acer negundo L. (Aceraceae), host trees with maturing seeds, and cluster on warm, sunlit surfaces prior to overwintering. We have recently shown that B. rubrolineata is attracted to the host-tree semiochemicals phenylacetonitrile and 2-phenethyl acetate. We report results of chemical analyses and laboratory bioassays suggesting that aggregation and sexual communication in B. rubrolineata are mediated by 2-phenylethanol. This compound serves as an aggregation pheromone for females, males, and 5th-instar nymphs in midsummer, and in males it appears to serve as a sex-attractant pheromone in early spring. As an aggregation pheromone, 2-phenylethanol originates from the feces of seed-feeding females and males and (or) the ventral abdominal gland of males. As a sex-attractant pheromone, it originates from the ventral abdominal gland of males that emerge from overwintering diapause. Aggregations of B. rubrolineata in the fall and winter are mediated by other as yet unknown pheromones.