Eight pheromones were collected as volatiles from living Dendroctonus beetles, and identified by GLC/MS. With Dendroctonus frontalis Zimmerman, in addition to earlier reported pheromones, the monoterpene ketone pinocarvone, and frontalin, were released by males, and the terpene alcohol myrtenol was released by both males and females. With D. ponderosae Hopkins, besides the previously known pheromones, endo-brevicomin and 3-methyl-2-cyc1ohexen-1-one (MCH) were released by males, and frontalin was collected from pairs of this species. Bioassay indicated that exo-brevicomin with D. ponderosae, and myrtenol as well as verbenone with D. frontalis, are multifunctional pheromones, since a small quantity (released by females) was synergistically attractive, and a larger quantity (released by males) was repressive and/or evoked “rivalry” behavior. For this latter effect, the male pheromones were designated antiaggregative-“rivalry” pheromones. With D. ponderosae, our data contradict the previous belief that this species releases no inhibitor and depends upon host-oleoresin effects to stop beetle aggregation. The identified male pheromones were released with males placed together, as well as with pairs, and the pheromones of fmale D. frontalis (though not of female D. ponderosae) were also released with females placed together, as well as with pairs. Both intrasex and intersex stimuli and response must be important in pheromone release, and may help answer certain questions about bark beetle behavior of practical significance in attempts to utilize these pheromones in control.