Fragments of Aspergillus nidulans DNA obtained after digestion with EcoRI, BamHI and HindIII endonucleases were cloned in Escherichia coli in plasmid pBR322. These gene banks were used for transformation of 15 E. coli auxotrophic mutants and in 5 cases prototrophic clones containing recombinant plasmids were selected. Three different recombinant plasmids conferring prototrophy to pyrF, proAB and argIF mutants were analyzed. Hybridization experiments indicated that in two of these plasmids the inserted fragment of DNA hybridized not only to the A. nidulans but also to the E. coli DNA.