Genetic variation was studied in unimproved grassland populations of two co
ntrasting outbreeding perennial grass species. A total of 27 populations of
Lolium perenne (perennial ryegrass) and 30 populations of Agrostis curtisi
i (bristle-leaved bent), sampled from seven and five regions spread across
southern Britain, were assessed at three and four isozyme loci, respectivel
y. The extent of genetic structure within and among populations was estimat
ed 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 observ
ed within L. perenne, a species of major agricultural importance. It is sug
gested that the absence from L. perenne of the pattern of genetic variation
found in A. curtisii is consistent with the occurrence of large-scale huma
n-mediated gene flow via 'improved' ryegrass cultivars. If this is the case
, then the disruption of natural patterns of genetic variation by the intro
duction of nonlocal genotypes may occur without apparent major ecological c
onsequences.