In order to investigate parasitoids of the genus Cotesia (Hymenoptera: Braconidae), larvae of a speciose group of butterflies, the tribe Melitaeini (Lepidoptera: Nymphalidae), were collected from several sites in Catalonia, northern Spain, a region that harbours ten out of the 20 European species of Melitaeini. New information on the natural history of the butterflies is presented, and the structure of their communities and patterns of larval parasitism are described. On the basis of mtDNA sequence data (COI gene), microsatellite data (ten loci) and behavioural experiments, we recognize seven biologically distinct species of Cotesia parasitizing the Melitaeini communities within this relatively small geographical area. In particular, the notional species C. melitaearum and C. acuminata each represents a series of cryptic species with narrow host associations. The possibility of direct competition among the parasitoids and/or indirect interactions between butterflies mediated by Cotesia parasitoids is explored. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 86, 45–65.