In order to evaluate the potential genetic resources of the wild relatives of crop plants, allozyme variation at 28 loci was determined for 28 Israel populations of Hordeum spontaneum, the progenitor of cultivated barley. Etectrophoretic properties of these loci and their variants are described. The enzyme loci exhibited a great range of polymorphism, from one to fifteen alleles per locus being detected. The average probability that two gametes drawn randomly from this collection would differ genetically at a locus was 0.19. The evidence indicates that natural populations of this species represent very rich reserves of genetic variability. The extent of differentiation between populations at each locus was compared with that shown by variation in spikelet morphology. The allozyme diversity was apportioned into 17% between regions, 32% between populations within regions, and 51% within populations. In contrast, spikelet variation occurred predominantly between populations. This indicated the species is highly differentiated in phenotype between regions, and presumably allozyme variation is involved in this differentiation. These results support field sampling and evaluation strategies which stress the sampling of more sites at the expense of reducing numbers per site.