The mountain pine beetle (Dendroctonus monticolae Hopk.) has killed millions of trees during the past ten years in Montana, Idaho, and eastern Washington. It was during a serious epidemic of this beetle that the writer undertook a study of the economic importance and biology of its enemies in an effort to determine what insects were the most beneficial and to work out their seasonal history so that artificial control work could be instituted, if possible, at a time that would destroy fewer beneficial insects.