Regulation of the two isozymes of 3-deoxy-D-arabino-heptulosonate-7phosphate synthase (DAHP synthase; EC 4.1.2.15) encoded by the genes ARO3 and ARO4 of Saccharomyces cerevisiae was studied. Both genes were shown to respond equally well to the general control of amino acid biosynthesis. Strains with mutations in these two genes were obtained by selecting first for a single aro3 mutation and afterwards for a double aro3 aro4 mutation. Gene ARO3, coding for the phenylalanine-dependent isozyme of DAHP synthase was cloned on the 2 micron multicopy vector pJDB207 by complementation of mutation aro3-1 in yeast. The ARO3 gene, carried originally on a 9.6 kb BamHI fragment (plasmid pME541A), was subcloned on a 1.9 kb HindIII-XbaI fragment (plasmid pME543). A transcript of about 1.5 kb was shown to proceed from the HindIII towards the XbaI site. Expression from the 9.6 kb as well as from the 1.9 kb fragment was normal on a multicopy vector, since in both cases DAHP synthase levels of about 50-fold the wild-type level were observed.