Variation in the length of simple DNA triplet repeats has been linked to phenotypic variability in microbes and to several human disorders. Population- level forces driving triplet repeat contraction and expansion in multicellular organisms are, however, not well understood. We have identified a triplet repeat- associated genetic defect in an Arabidopsis thaliana variety collected from the wild. The Bur- 0 strain carries a dramatically expanded TTC/ GAA repeat in the intron of the ISOPROPYL MALATE ISOMERASE LARGE SUB UNIT1 ( IIL1; At4g13430) gene. The repeat expansion causes an environment-dependent reduction in IIL1 activity and severely impairs growth of this strain, whereas contraction of the expanded repeat can reverse the detrimental phenotype. The Bur- 0 IIL1 defect thus presents a genetically tractable model for triplet repeat expansions and their variability in natural populations.