Within an egg gallery of the mountain pine beetle (Dendroctonus ponderosae Hopk.) only one male is found with one female even though the virgin female or its frass may have been equally attractive to more than one male. To ascertain how the number of males joining a female is determined, a series of 10 lodgepole pine rearing slabs and 10 observational plates (Reid 1962) were infested with virgin mountain pine beetle females. The females were allowed 1 day to become established prior to the experiments. On the second day one virgin male was placed in each gallery. The next day a second was placed near the entrance to the gallery and its behavior was noted, as well as that of the first male.