The extranuclear mitochondrial oligomycin-resistant mutation of Aspergillus nidulans, (oliA1), was transferred asexually into four nuclear oligomycin-resistant strains of different phenotypes. In all four cases, the possession of the nuclear plus extranuclear mutation led to an increase in the in vivo level of oligomycin resistance. In two cases, the altered cytochrome spectrum and impaired growth ability determined by (oliA1) were suppressed by the nuclear mutations. In the third case, the in vitro oligomycin resistance of the double mutant ATPase was dramatically increased above that of either of the component single mutant strains, indicating a synergystic interaction between the nuclear and extranuclear gene products. In the fourth case, the double mutant became cold-sensitive. A new extranuclear mitochondrial oligomycin-resistant mutation (oliB332) is described. This mutant is phenotypically similar to, though not identical with, (oliA1) but is separable by recombination. A range of nuclear oligomycin-resistant mutants have been mapped. Despite presenting five distinctly different phenotypes, they all map at the same locus.