Laboratory bioassay demonstrated that the pheromone (Z)-6-heneicosen-11-one, its (E)-stereoisomer, and an epoxide (Z)-6,7-epoxyheneicosan-11-one are all attractive to the male Douglas-fir tussock moth,Orgyia pseudotsugata(McDunnough). The pheromone was approximately 27 times more attractive than the (E)-isomer and 300 times more attractive than the epoxide. An alcohol, (Z)-6-heneicosen-11-ol, was found to have a weak inhibitory effect on pheromone response.In the field, dosages of synthetic pheromone as low as 10-ng captured males and 50-ng baits were still attractive after 7 days' exposure. A controlled-release formulation, emitting pheromone at an estimated rate of 4 ng/min at 25°C, approached but did not equal the attractiveness of a live female. It is likely that an additional compound is present in the natural pheromone that is required to optimize response to the synthetic preparation.(Z)-6-heneicosen-11-one is also an effective attractant for the western tussock moth,O. canaEdwards, the rusty tussock mothO. antiqua(Linn.),Dasychira vagans grisea(Barnes and McDunnough) andD. grisefacta ellaBryk.