Internal Transcribed Spacer (rDNA) data have been obtained from 193 terrestrial orchid species, encompassing all genera and the great majority of the widely recognized species of Orchidinae, a heterogeneous selection of species of Habenariinae, and a single Diseae outgroup. The resulting parsimony-based phylogeny reveals 11 well-resolved clades within Orchidinae and confirms the triphyly of Orchis s.l. Relationships within Habenariinae are more ambiguous; the subtribe appearing weakly monophyletic or paraphyletic according to the tree-building algorithm used, though the genus Habenaria is clearly highly polyphyletic. Species-level relationships are satisfactorily resolved within most of the major clades, the exceptions being widely recognized as taxonomically 'critical' groups such as Serapias, derived Ophrys, and the former genus 'Nigritella'. Relationships among the 11 major clades are less clear, and are currently being further investigated by combining ITS with new trnL sequence data for a carefully selected subset of 63 species. Current phylogenies support the recently recognized 2n = 36 clade; they also indicate a 2n = 40 clade that is further diagnosed by digitate root-tubers, and is derived relative to the basally divergent clade of exclusively Asian genera. This Asian clade identifies the plesiomorphic chromosome number as 2n = 42, and is consistent with possible southeast Asian origins of several of the more basally divergent major clades within the subtribe and of the subtribe itself, though ambiguous phylogenetic placements of the potentially pivotal African genera Stenoglottis and Brachycorythis also permit an African origin. Morphological characters informally 'mapped' across the molecular phylogeny and showing appreciable levels of homoplasy include floral and vegetative pigmentation, flower shape, leaf posture, gynostemium features, and various pollinator attractants. Brief scenarios, some adaptive and others non-adaptive, are postulated for specific evolutionary transitions.

Key words: ITS, karyotype, morphology, Orchidaceae, Orchideae, trnL