The world of economic entomology has known in a general way that Aphelinus mali Hald., a North American parasite of the woolly root-louse of the apple (Schizoneura Eriosoma lanigenim Hausm.) has during the past nine years been carried to many parts of the world and that in some places it has been of great help to apple-growers. But the whole story has not been told, and only a few know it in more than a fragmentary way. This is an attempt to bring all the facts together as a contribution to the rapidly growing literature referring to natural control.