The antiaggregative pheromone 3-methyl-2-cyclohexen-1-one (MCH) of the Douglas-fir beetle (Dendroctonus pseudotsugae Hopk.) was diffused at three rates and three spacings around attractive felled trees. Optimum treatment was ca. 1–2 mg/day at 10-ft spacing (0.06–1.3 g/acre per day), which reduced Douglas-fir beetle attacks and progeny by 96% and 91%, respectively. Progeny in trees exposed to higher than optimum MCH concentration were less mature, but not significantly fewer than progeny in controls. Densities of immature stages of nine other taxa of insects, both entomophagous and commensal, were determined. Abundance of larvae of the predacious clerids Enoclerus sphegeus Fab. and Thanasimus undatulus Say was significantly correlated with abundance of Douglas-fir beetle attacks and progeny. The predator Temnochila chlorodia (Mann.) and the associate Pseudohylesinus nebulosus (Lee.) were more dense on samples in the test area where Douglas-fir beetle population and damage were lowest. Density of the dipterous predator Medetera aldrichii (Wheeler) was correlated with numbers of beetle entrances, but decreased at high MCH concentrations. Abundance of Coeloides brunneri Vier., a braconid parasite, was correlated with numbers of beetle brood. Presence of MCH appeared to increase abundance of the associate Pissodes fasciatus Lec. and modify its distribution in trees. Use of methylcyclohexenone for preventing infestation of susceptible trees is a potential control strategy, but a more practical and effective formulation must be developed.