1. Caterpillars of the myrmecophilous butterfly Maculinea rebeli showed strong evidence of contest competition when introduced at high densities to laboratory nests of Myrmica ants. 2. This is attributed to the direct feeding of caterpillars by workers, which select a few individuals to nurture when food or ant numbers are limiting. It contrasts with published data for a congener, Maculinea arion, which has predacious larvae and experiences scramble competition in crowded ant nests. 3. Worker ants from two Myrmica rubra colonies (I and II) were used to found the laboratory nests hosting Maculinea rebeli. Nests from each source reared a similar biomass of Maculinea, but whereas those containing M. rubra I workers reared eight to ten lightweight caterpillars each, cultures from colony II reared half as many caterpillars, each of about double the weight. 4. Differences in nest capacity may be due to the different social structures of colonies I and II at the start of the experiment.