An electrophoretic karyotype of Aspergillus nidulans has been obtained using contour-clamped homogeneous electric field gel electrophoresis. Six chromosomal bands were separated, with two of the bands migrating as doublets. Using the Schizosaccharomyces pombe and Saccharomyces cerevisiae chromosomes as size standards, we estimate the sizes of the chromosomes to be between 2.9 and 5.0 megabase pairs (mb) with a total genome size of approximately 31 mb. Four of the eight genetic linkage groups were assigned to chromosomal bands by hybridization of contour-clamped homogeneous electric field gel blots with various radiolabeled probes each specific to a particular linkage group. Contour-clamped homogeneous electric field gel analysis of reciprocal translocation strains gave chromosomal assignments for the four remaining linkage groups. In order of decreasing size, the A. nidulans chromosomes are: VIII (5.0 mb), VII (4.5 mb), II (4.2 mb), I and V (3.8 mb), III and VI (3.5 mb), and IV (2.9 mb).