The toxicity of pine resin vapors to adult bark beetles was selected as one means of investigating the resistance of pines to these insects. When adults of Dendroctollus brevicomis Lec. were confined to a saturated atmosphere in a 30-cc. test tube, mortality rates differed significantly between species of resins and between resin and turpentine (the liquid commercial derivative of resin). In a 150-cc. screw-cap jar fumigation chamber, D. brevicomis and D. jeffreyi Hopk. were able to tolerate the volatile fraction of their natural host resin, respectively Pinus ponderosa Laws. and P. jeffreyi Grev. and Balf; but neither beetle was able to tolerate the vapors from the other's host resin. With increased time of exposure, neither beetle was able to tolerate the saturated vapors of resin from a P. jeffreyi ✕ ponderosa hybrid. This led to the hypothesis that bark beetles can tolerate the resin vapors of their hosts but cannot tolerate that of a nonhost.