A series of grasslands of different ages which have suffered copper pollution for different lengths of time have been used to follow the evolution of metal tolerance.Species diversity in the grasslands decreases with increasing pollution, so that in the most polluted sites only Agrostis stolon(fera, sometimes with Agrostis tenuis, is to be found. In the polluted sites there is a great deal of bare ground in the youngest (5 years old) grasslands but complete cover in the oldest grasslands (70 years old).The A. stolonjfera populations in unpolluted sites are not copper tolerant. But in the polluted sites there is an increase in the tolerance of individuals, and an increase in the frequency of tolerant individuals, as the age of the population increases: even the youngest population shows considerable tolerance.An analysis, using morphological and esterase isoenzyme variation as criteria, shows that the tolerant populations consist of a large number of different individual clones or genotypes. The populations must have evolved by the rapid selection of a large number of the tolerant or partly tolerant individuals which can be shown to exist in unpolluted populations at very low frequency.The selection can be shown to occur at both seedling and adult stages, and to cause the evolution of tolerance effectively in a single generation.