The aspartase gene (aspA) of E. coli was isolated in 2 plasmids, pGS73 and pGS94, which contain segments of bacterial DNA (12.5 and 2.8 kilobase, respectively) inserted into the tet gene of the vector pBR322. The plasmids were constructed by sequential sub-cloning from a larger ColE1-frd+ hybrid plasmid. The location of the aspA gene confirmed predictions based on a correlation between the genetic and restriction maps of the corresponding region. The aspartase activities of plasmid-containing aspA mutants were amplified 4- to 6-fold relative to aspA+ parental strains. The aspA gene product was tentatively identified as a polypeptide of MW 55,000 which is somewhat larger than previous estimates (MW 45,000-48,000) for aspartase (EC 4.3.1.1).