Normally sheep’s fescue (Festuca ovina) carries a single locus encoding the cytosolic enzyme phosphoglucoisomerase (PGI). However, at several localities in southern Sweden plants with more than two different alleles have been found by isozyme studies. Crossing experiments and subsequent progeny analyses revealed two unlinked loci. The additional locus was polymorphic for two alleles with different electrophoretic mobilities. Two of the numerous alleles at the primary locus had the same mobility. A complex gene coding for both electrophoretic types was also found to map to the additional locus, but no such complex gene has been found at the primary locus. The genes at the additional locus can be lacking, or present in hemizygous or homozygous form. Allozymes encoded by the additional locus readily form heterodimers with those encoded by the primary locus, thus giving rise to multibanded isozyme patterns. This segregating, polymorphic gene duplication may well represent a rare case where the initial stage of the evolution of a new gene can be directly studied.