Abstract.1. A laboratory competition experiment is described in which the growth and development rates of larvae of the damselflyIschnura elegans(Lind.) were measured over an entire instar.2. Two larval instars which commonly occur together in the field were used in the experiment; they were maintained with a superabundance of prey and either larvae from the same or the larger/smaller instar.3. Small larvae suffered increased development times and decreased size increases at the moult in the presence of large larvae but similar interference effects were not evident when these smaller larvae were in the presence of other small larvae.4. Development time and size increases of large larvae were not significantly affected by the presence of small larvae.5. Irrespective of the instar combinations investigated, interference effects were reduced when there were more perches available, although in only a few cases was this reduction significant.6. The consequences of the asymmetric competition reported in the experiment for the study of lifetime reproductive success in damselflies are discussed. Late emerging adults may incur reduced reproductive success.