Data are given on the genetic structure, morphology and chorology of the Long-Spurred orchid,Orchis longicornuPoiret (1789) from Sardinia and the occurrence in this island of the morphologically similarOrchis morioL. (1753), often recorded for Sardinia, is investigated. The genetic analysis of 27 enzyme loci in population samples from locations where both species had been recorded showed that: (i) in the population samples from Sardinia the polymorphic loci are in Hardy-Weinberg equilibrium; (ii) these samples are genetically poorly differentiated from each other (average Nei'sD= 0.01): less than 2% of the overall genetic variation observed is attributable to differences between populations (GST= 0.015); (iii) a high rate of gene flow was estimated between Sardinian populations:Nm≈ 4, possibly owing to seed dispersal by wind; (iv) when Sardinian samples are compared withO. moriofrom continental Italy, significant differences in genetic variation were observed: average He= 0.16 in the former, 0.12 in the latter; (v) the genetic distance found between Sardinian populations and those ofO. moriofrom continental Italy is relatively high: average Nei'sD= 0.18, average Rogers'D= 0.22; (vi) highly significant differences in allele frequencies were found at a number of loci (Mdh-2, Sod-3, Pgm-1, Gpi-1, Gpi-2) between populations from Sardinia and continental Italy, each giving a probability of correct identification that varies from 0.90 to 1 (diagnostic loci). These findings provide evidence thatO. moriois not present in the material genetically analyzed from Sardinia, which includes a single species, apparently corresponding toO. longicornu.The slight heterogeneity observed at some loci in the Sardinian population samples can be attributed in some cases (e.g.Mdh-1, Adh, Est-6) to local differences, possibly adaptive; in others (e.g.Pgm-1andGpi-1) to genetic drift effects. The hypothesis that some alleles (Gpi-1100,Dia101,NADHdh-296) found inO. morioand recorded at low frequency in a few Sardinian populations reflect palaeointrogressive phenomena (owing to sporadical immigrant individuals ofO. moriohaving diluted their genes inO. longicornugenome through multiple generations of backcrosses) is not sufficiently supported by the available data. The genetic data are in agreement with the results from morphological studies. These involved the examination of about 1500 specimens (both alive and dried) from all over Sardinia, and their comparison with herbarium specimens ofO. longfcornufrompatria typica(Algeria) and ofO. moriofrom continental Italy. The typification ofO. longicornuis given. The specimens from Sardinia correspond well, on a morphological basis, to the Algerian ones and therefore were all assigned toO. longicornu.Several characters were considered, differentiatingO. longicornufromO. morioboth in fresh and dried specimens. However, some of them, currently used in diagnostic keys, show much overlap between the two species. This explains the quotations ofO. moriofrom Sardinia, that appear to be misidentifications ofO. longicornu.A description ofO. longicornufrom Sardinia is provided, and its distribution in the island is defined, on the basis of (i) a critical revision of literature records, and (ii) the examination of fresh and dried Sardinian specimens.O. longicornuis shown to be widespread throughout Sardinia, living in many different habitats from 0 to 1500 m a.s.l.