The growth-rates and chromosome numbers of plants taken from a natural population of hybrids between perennial rye grass and meadow fescue were determined. The growth-rates of diploid hybrids resembled that of rye grass while those of the triploids were similar to that of meadow fescue. Experiments on plants grown singly and in competition showed that fescue was adversely affected by low temperature and rye grass by low light and also by the reduced nutrient supply arising from competition. The persistence of the sterile hybrids is explained by their recurrent formation coupled with greater longevity and vegetative spread, arising from growth characteristics intermediate between those of their parents. The persistence of rye grass and the disappearance of fescue when periodically grazed appear to result from the earlier growth and more rapid rate of tillering of the former. However, rye grass has fewer and smaller meristems available for regrowth if allowed to grow until anthesis, and under these conditions fescue dominates. The intermediate growth characters of the hybrids allow them to survive spring grazing more readily than meadow fescue and cutting for hay more readily than rye grass.