Saccharomyces cerevisiae, Candida maltosa and Pichia guilliermondii have been transformed by the plasmid pYe(ARG4)411, which contains the S. cerevisiae ARG4 gene inserted into pBR322. In all transformants argininosuccinate lyase as well as beta-lactamase were detected. The ARG+ phenotype of transformants is mitotically unstable. Closed circular pYe(ARG4)411 DNA was detected in transformant DNA preparations by hybridization to pBR322 DNA and by transformation of E. coli to ampicillin resistance.