The mitochondrial DNAs (mtDNAs) and the ribosomal repeat unit (ribosomal DNA, rDNA) of blackAspergillusisolates collected in various parts of the world were examined. Wide-ranging mtDNA variation was observed in natural populations of theAspergillus nigeraggregate. Most isolates were classifiable asA.nigerorAspergillus tubingensisaccording to their rDNA and mtDNA patterns. The mtDNA variation was distributed unevenly in the populations studied. The mtDNAs of most of the isolates collected in Australia were of theA.tubingensistype, with an unexpectedly high degree of variation, while the rDNA of these isolates exhibited the sameA.tubingensispattern as that of isolates from other locations. Some other local populations displayed very little polymorphism in their mtDNA and rDNA. Hybridization experiments in which clonedA.nigerandAspergillus nidulansmtDNA fragments were used revealed that the two main mtDNA groups corresponding toA.nigerandA.tubingensisare more distantly related than concluded earlier. Six of the 13 Brazilian isolates examined exhibited mtDNA and rDNA types different from those of all the other strains and could not be classified into the above species. Classical taxonomic examination of these strains is in progress.Key words:Aspergillus nigeraggregate, mitochondrial DNA, RFLP.