Genetic variation was studied in unimproved grassland populations of two contrasting outbreeding perennial grass species. A total of 27 populations of Lolium perenne (perennial ryegrass) and 30 populations of Agrostis curtisii (bristle-leaved bent), sampled from seven and five regions spread across southern Britain, were assessed at three and four isozyme loci, respectively. The extent of genetic structure within and among populations was estimated using unbiased F-statistics. In A. curtisii, a nonagricultural species, populations from adjacent regions were found to be more genetically similar than those separated by greater distance. The reverse situation was observed within L. perenne, a species of major agricultural importance. It is suggested that the absence from L. perenne of the pattern of genetic variation found in A. curtisii is consistent with the occurrence of large-scale human-mediated gene flow via 'improved' ryegrass cultivars. If this is the case, then the disruption of natural patterns of genetic variation by the introduction of nonlocal genotypes may occur without apparent major ecological consequences.