Summary 1The grass Festuca ovina is an important constituent of the species-rich`alvar' grasslands on the Baltic island of O È land. Levels of allozyme polymorphism are high and variation is known to be correlated with habitat variation (soil moisture, pH and depth). 2 A 9-year ®eld experiment on species diversity provided replicate plots (in three sites) that had been subjected to six di erent experimental treatments (control; N P K; P K; N K; K; water). 3 Samples of F. ovina were collected and analyses of deviance were used to investigate associations between allele frequencies, at each of four polymorphic loci, and the nutrient/water treatments. We also used the models to estimate predicted values for the alleles in di erent nutrient/water treatments and in interactions involving the nutrient/water treatments and additional explanatory variables (vegetation height and clipping). 4 There were signi®cant allozyme frequency di erences between samples of F. ovina from the six di erent nutrient/water treatments in the grassland experiment. Frequencies in the fertilized or watered plots had diverged from those in the control plots. There were also signi®cant allele±habitat associations (after the removal of site e ects), especially at the Pgi-2 locus. 5 Soil moisture was the only variable that was common to this study and an earlier study of variation in F. ovina in natural habitats. In natural populations, the Pgi-2-2 allele was signi®cantly associated with soil moisture and was more common in dry habitats. Our ®ndings that the frequency of the Pgi-2-2 allele was signi®cantly a ected by the nutrient/water treatments, and that it was rarest in the treatment that involved the addition of extra water, were therefore as predicted. 6 The study supports the conclusion, from an earlier study of populations in unmanipulated grassland habitats, that selection is contributing to the ®ne-scale patterning of genetic variation in the alvar populations of F. ovina.