A PCR-based technique, involving the random amplification of polymorphic DNA (RAPD), was used for assessing genomic variability among a wide range of culture collection strains of black Aspergilli and related species. The performance of this technique is compared with that of the two other genetic techniques most commonly used, namely restriction fragment length polymorphisms on rDNA and isozyme analysis. The eight main groups as assigned by RFLP were also distinguished by RAPD patterns. On the basis of 122 polymorphic RAPD products using six random primers, the 17 collection strains examined could be subdivided into 15 distinct sub-groups. We suggest that the RAPD method is a quick and reliable tool for establishing the amount of genetic variability in closely related species. Our study indicates that the complex group of black Aspergilli is characterized by a high degree of genetic differentiation. This is also apparent from the considerable karyotype variation present in the group.