We have constructed a phage, lambda Ncl, which comprises a 4.0 kb HindIII insert of Neurospora DNA into the immunity region of the vector lambda 598. lambda Ncl complements the aroD6 mutation of E. coli, permitting the formation of galaxy plaques on medium lacking aromatic supplements, and transforms an aro-9 qa-2 Neurospora mutant to prototrophy at a low frequency. Low levels of 5-dehydroquinate hydrolyase (E.C.4.2.1.10.), with properties unlike those of the catabolic isoenzyme that is coded by qa-2, are present in E. coli aroD6 cell lysates following infection with lambda Ncl. lambda Ncl does not hybridize with qa-2 DNA and it is concluded that it contains at least the aro-9 region of the pentafunctional aro cluster gene.