A gene transfer system originally developed for Fusarium oxysporum has been applied to seven species of filamentous fungi of agricultural and industrial importance. This transformation system relies on the selection of mutants deficient in nitrate reductase by positive screening. Such mutants were recovered easily in all the fungi tested--without mutagenic treatments--through their resistance to chlorate. They were transformed by a plasmid vector (pAN301) carrying the Aspergillus nidulans wild-type gene (niaD). Transformation frequencies ranged from one to ten transformants/micrograms plasmid DNA. The general properties of the transformants were analyzed. Most of them are mitotically stable, and the integration of the vector into the host genome frequently occurred in a tandem fashion.