This paper provides a genetic map of Aspergillus niger. At present 84 markers have been assigned to eight linkage groups. The chromosomal location of 60 markers is presented in this paper. The allocation of markers is based on recombination due to mitotic crossing over. Various methods for selection and analysis of homozygous recombinants were applied, using colour, auxotrophic and resistance markers. In addition, transformants carrying the heterologous Aspergillus nidulans gene coding for acetamidase (amdS) were used for mitotic mapping of markers in several linkage groups. In most of the transformants the amdS insert appeared to be centromere-distal to all known genetic markers, thus extending the genetic map. The linear order of the markers in the eight linkage groups has been determined. On the basis of these and earlier experiments tentative genetic maps for the eight linkage groups are presented. Genetic markers were found on both arms of the chromosomes, except for chromosomes II and IV. The genetic distance between markers and the centromere varies from about 10(-4) (LG I, II, V) up to more than 10(-2) (LG III, VI, VIII). The total frequency of mitotic recombination per genome in this fungus has been estimated to be at least 1.2 x 10(-1).