Island biogeographical theory predict species richness to increase with habitat area and to decrease with isolation from colonisation sources. This theory has been applied to habitat fragments, and the predictions tested and found valid in empirical studies on fragments of deciduous woodland in northern Europe. However, previous results on fragments of grassland have been ambiguous. In the present study the species richness of vascular plants was investigated in 63 patches of grassland and heathland scattered in an agricultural and forested landscape using multiple linear regression. The relationship between species richness and patch area, isolation and local habitat conditions including heterogeneity were examined. Area was an important determinant of species richness, both in the full data set and in a subset of small habitat patches. In contrast, spatial isolation and habitat heterogeneity were not important factors determining species richness. Differences in soil acidity were accounting for a large proportion of the variance in species richness. This result is probably due to differences in the size of the regional species pools of grasslands with different levels of soil pH.