Aspergillus niger C was treated with UV and gamma irradiation (60Co), nitrogen mustard and colchicine. Mutagenic treatments resulted in substrains with either increased or decreased citric acid production, while some were unchanged in this respect. Low-yielding strains predominated in all the experiments. Analysis of the superior substrains showed gamma irradiation to be most effective in producing the greatest number of superior mutants. © 1972 Institute of Microbiology, Academy of Sciences of the Czech Republic.