We have examined the distribution of trinucleotide repeats in the yeast genome. Perfect and imperfect repeats, ranging from four to 130 triplets were recognized and the repartition of different triplet combinations was found to differ between Open Reading Frames and Intergenic Regions. Examination of different laboratory strains, revealed polymorphic size variations for all perfect repeats studied, compared to an absence of variation for the imperfect ones. Size variations were found discrete in the range of 6-18 triplets, each strain showing one allelic form for a given repeat array. The distribution and stability of trinucleotide repeats in the yeast genome resembles that of humans and may provide an experimental approach to study the mechanisms of their expansion.