Phylogenetic relationships among the Chiastocheta species (Diptera, Anthomyiidae) were investigated using a 1320-bp mitochondrial DNA fragment including parts of cytochrome oxidase subunits I and II. Larvae of the Chiastocheta genus feed exclusively on Trollius (Ranunculaceae) seeds. Six Chiastocheta species (and two vicariant taxa) coexist on Trollius europaeus in Europe, where they were shown to be the sole pollinator of their host plant. In Asia, several Trollius species are parasitized by Chiastocheta spp., and several other Trollius species are free of Chiastocheta. The plant-insect association is thus obligate in Europe but facultative in Asia. This system therefore provides a unique opportunity to study the evolution of an obligate mutualism between a plant and its seed parasite pollinators and its consequences on insect diversification. Most parsimonious, distance, and maximum likelihood analyses of 16 haplotypes from Europe and 14 from Asia show that European species do not form a monophyletic group. The species relationship indicated by egg morphology is only partly supported by molecular data. Moreover, a strong discrepancy between nucleotide variation and male genitalia morphology variation was found, particularly within and among European species coexisting on a single host plant. Molecular divergence across species ranges from 0 to 4.6%, suggesting a recent origin of the genus and several events of diversification, involving both host shifts and within-host-plant radiation. We discuss alternative evolutionary scenarios that are consistent with molecular data. ©2002 Elsevier Science (USA).