Genetic structure of 120 wild populations of Lolium perenne and 50 populations of L. rigidum was studied using starch gel electrophoresis. Allelic frequencies were obtained from 12 polymorphic isozyme loci. Gene diversity indices (number of alleles (A), observed (Ho) and expected (He) heterozygosity) were significantly higher in L. rigidum (A=3.13; Ho=0.369; He=0.405) than in L. perenne (A=2.72; Ho=0.308; He=0.322). For both species, most of the diversity appeared to be within populations (GST=0.110 and 0.170 for L. perenne and L. rigidum, respectively). Despite this weak genetic differentiation, significant patterns of geographical variation for diversity indices and allele frequencies were observed in L. perenne populations; the three genetic indices (A, Ho, He) showed the same trend of variation, with the lowest values in the north-west part of the distribution area (United Kingdom, Ireland) and the highest ones in the south-east (Turkey, Lebanon, Cyprus, Iraq, Iran). In the same way, as indicated by logistic regression analyses between allelic frequencies and geographical data of L. perenne populations, the latitudinal gradient of allelic frequencies appeared to be more pronounced, although significant relationships also existed with longitude. In contrast, no spatial organization of the diversity was detected in L. rigidum. Hypotheses concerning the taxonomic relationships and the genetic and geographical origins of the two species are discussed. Lolium perenne could be derived from a small bottleneck of L. rigidum populations in the Middle East, and its present distribution area in Europe could be explained either by the extension of primitive agriculture from the fertile crescent, or as a consequence of postglacial recolonization from southern refugia.