Volatiles produced by mountain pine beetles (MPB), Dendroctonus ponderosae, were identified by GC/MS from air passed over emergent males, fed males, and the frass of virgin females reared from lodgepole, ponderosa, white, and sugar pine. exo- and endo-Brevicomin were released by males from all four hosts. Unfed males from lodgepole pine also released diacetone alcohol; unfed males from white pine released frontalin. The frass of females from all four hosts released trans-verbenol, linalool, borneol, verbenone, myrtenol, and piperitone. Other volatiles such as cis-verbenol, trans-pinocarveol, terpinen-4-ol, alpha-terpineol, octanone, and 2-p-methen-7-ol, were detected from frass of MPB reared from certain hosts but not others. Results of cross-feeding experiments between MPB from white and lodgepole pine indicated that differences in frass volatiles among the populations studied result from the monoterpene composition of the host pine diet rather than from physiological differences among populations. Males reared from and fed in all four hosts released frontalin and various other volatiles. In field bioassays, 3-caren-10-ol changed the sex ratio of MPB trapped to favor males, and pinocarvone reduced the response to 48% of the attractant control. Linalool, piperitone, trans-pinocarveol, acetone, and ethanol did not affect trap catches at the evaporation rates tested. exo-Brevicomin was inhibitive at the highest concentration tested and had no affect on aggregation at lower evaporation rates down to < ppm. (-)-trans-Verbenol and myrcene attracted more male MPB than (+)-trans-verbenol and (+)-alpha-pinene, respectively; but females were attracted equally. A high concentration of frontalin inhibited aggregation, but a low concentration (100 ng/h) neither inhibited nor enhanced aggregation.