Control of leaf shape and size was studied at the molecular level by a mutational approach in Arabidopsis thaliana. Mutations that affected leaf growth allowed the identification of genes that are important in that morphological process. The Activator/Dissociation (Ac/Ds) transposable element system of maize was modified to be used as mutagen of the Arabidopsis genome. We isolated the drl 1-2 mutation that is characterized by narrow leaf blades. We describe the mutant phenotype and have cloned and analyzed the gene. These combined analyses will give insight into the molecular mechanisms that control leaf growth.