Abstract
1. The oviposition behaviour of the ichneumon flies- Horogenes chrysostictos and Nemeritis canescens has been examined in relation to their multiparasitism of larvae of the moth Ephestia sericarium. 2. Horogenes is able to distinguish healthy hosts from those that are parasitized by Nemeritis, both when given a simultaneous choice between them and when exposed to each alternately.