A description is offered below for an unusual species of Pimplopterus recently reared from the larch twig borer, Argyresthia laricella Kft., in several localities in Eastern Canada. I am indebted to my associate Mr. C. D. F. Miller, who fist drew my attention to the species and the peculiar features of its ovipositor, and to officers at several Forest Biology laboratories in Canada, who have kindly provided an excellent series of reared specimens.