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. cana Edwards, the rusty tussock moth O. antiqua (Linn.), Dasychira vagans grisea (Barnes and McDunnough) and D. grisefacta ella Bryk.