Intraspecific protoplast fusion was carried out between different citric acid-producing strains of Aspergillus niger. Protoplasts prepared from the mycelia of auxotrophic mutants were treated in 30% (w/v) polyethylene glycol solution containing 0.01 M CaCl2 and 0.5 M KCl, and fusion frequencies were 2.0-6.3%. Fusants were heterokaryons and in some cases formed sectors of the prototroph. Heterodiploids were induced from a heterokaryon by d-camphor treatment. Citric acid productivity of one heterodiploid was intermediate between those of parent strains in both shaking and solid cultures.