The known differences in response byDendroctonus pseudotsugaeHopkins to its pheromone 3-methyl-2-cyclohexen-1-one (MCH) correspond with differences in concentration of the substance. When combined with other pheromones ofD. pseudotsugaeand host-plant volatiles, a low concentration of MCH elicits increased arrestment and stridulation of males and increased flight attraction of both sexes, whereas a high concentration of MCH inhibits the normal positive response of both sexes to these other pheromones and host plant volatiles. Since differential release of MCH apparently occurs during natural beetle behavior, these effects are suggested as multiple functions of the pheromone.