Rudinsky (1969) and Rudinsky et al. (1973) proposed that 3-methyl-2-cyclohexen-1-one (MCH) is produced by the female Douglas-fir beetle,Dendroctonus pseudotsugaeHopkins, and released by the female in response to male stridulation. MCH is an effective deterrent to Douglas-fir beetle attack on cut Douglas-fir trees (Furniss et al. 1972, 1974). On the basis of these studies a novel chemoacoustic communication system has been suggested (Rudinsky et al. 1973) by which an attractive Douglas-fir beetle female instantaneously “masks” her own chemical attractants with an “anti-attractant” pheromone (Furniss et al. 1972) as soon as a sonic signal has been received from the male or from an electronic playback of male sounds. The purpose of our study was to explore the sequence in which male and female Douglas-fir beetles produce and release seudenol (Vité et al. 1972) and the attractant inhibitor MCH. The methodology used is described elsewhere (Vité and Renwick 1970).