The aggregation pheromone ofDendroctonus jeffreyiwas investigated by combined gas chromatography and mass spectrometry of volatiles from hindguts of attacking beetles. The major compounds in females were identified as 1-heptanol and 2-heptanol, which are produced by oxidation of heptane present in the host resin. In field tests, 1-heptanol in combination with heptane was attractive, but racemic 2-heptanol was not. The addition of 2-heptanol to 1-heptanol appeared to reduce the number of beetles responding.The results indicate a probable difference between the chemical communication systems ofD. jeffreyiandD. ponderosae, both of which have apparently adapted to utilize the metabolic products of potentially toxic hydrocarbons in their hosts.