The morphologically diverse annual sweet vernal grasses, Anthoxanthum aristatum and Anthoxanthum ovatum, have been traditionally classified as separate species, each containing several infraspecific ranks. However, transitional forms between the species have been commonly found in sympatric populations that present a remarkably high morphological variability. Alternative hypotheses based on hybridization events between these two polymorphic Mediterranean species, or the existence of only one highly variable taxon, have been suggested as potential explanations for the presence of intermediate forms. In this study, we aimed at disentangling whether the integrity of the species is maintained in sympatric scenarios and at clarifying the taxonomic boundaries of the taxa described within this complex. For this, we analysed macro- and micromorphological and genetic data from 12 Iberian populations of A. aristatum and A. ovatum. Our results revealed the existence of two lineages in the A. aristatum/A. ovatum complex that were not consistent with the morphological circumscription of these species and suggested introgression between A. aristatum and A. ovatum. We also observed that morphological variability was not equally distributed between the lineages. Finally, we found some morphological and genetic support for the specific recognition of A. aristatum and A. ovatum, but the definition of any infraspecific category within these taxa does not seem to be justified. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 164, 53–71.