Monoterpenes present in grand fir,Abies grandis(Douglas) Lindley, phloem were tested for their effects on fir engraver,Scolytus ventralisLeConte, adults and the symbiotic fungusTrichosporium symbioticumWright. All of the monoterpene vapors caused significant beetle mortality within 4 h. Toxicity was observed at doses normally found in the host tree, either in unattacked phloem or in reaction tissue induced by the pathogen. Incorporation of any monoterpenes into the growth medium, at doses normally found in the host tree, substantially reducedT. symbioticumgrowth. Limonene was the most inhibitory synthetic compound, but grand fir resin had a greater effect. This suggests that other factors are also involved in fungal inhibition.