The cyanogenic phenotypes of 19 populations of adult Trifolium repens plants were determined. Substantial differences were found, both in the frequency of Ac- as in Li-phenotypes. The differences found are not attributable to large climatic differences, but to factors acting on a local scale. Ac and Li are not independently assorted: six out of sixteen populations studied showed a positive linkage disequilibrium. As Ac and Li are not genetically linked, the association must be the result of epistatic selection favouring Ac-Li-phenotypes. The relative importance of deterministic forces, i.e. natural selection, and stochastic ones (founder effects, drift) in determining the phenotypic frequencies found in The Netherlands are discussed.