Samples of .apprx. 100 plants from each of 22 populations of L. perenne representing 15 cultivars, and from 13 populations of L. multiflorum representing 6 cultivars were scored for isozyme variants in 5 enzyme systems: PGI, GOT, ACP, PGM and 6-PGD [phosphoglucoisomerase, glutamate oxaloacetate transaminase, acid phosphatase, phosphoglucomutase, 6-phosphogluconate dehydrogenase]. From the individual banding patterns a genetic interpretation of the variation was formulated and population studies of the resulting 6 polymorphic enzyme loci were performed. No strong indications of partial selfing was found since at 4 of the 6 loci, Pgi 2, Got 3, Pgm 1 and Pgd 1, the genotypic proportions were in correspondence with the Hardy-Weinberg expectations. This indicated, further, that the genetical interpretations of the banding patterns might be correct. Deviations from Hardy-Weinberg proportions for Acp 1 and Got 2 indicated presumably selection working on the linkage group including these loci. Gametic phase disequilibrium was observed between Pgi 2 and Pgd 1 for populations of 1 cultivar. These results were discussed in relation to the variation expected within a cultivar.