Abstract
The cecidomyiid Endaphis sp. is a locally common endoparasite of the aphid Euceraphis gillettei on alder in coastal British Columbia. The midge represents the first known protelean parasite of aphids that may develop gregariously inside the host. Field and laboratory observations are presented on the parasite’s life history and host instar preference. Mechanisms enabling the partitioning of the available “host space” between Endaphis sp. and the aphidiid parasite Praon sp. are discussed.