The electrophoretic karyotype of 17 antagonistic and pathogenic strains of Fusarium spp. has been established by using contour-clamped homogeneous electric field gel electrophoresis. Intact chromosomal DNA was prepared from fungal protoplasts with standard procedures. Up to 11 distinct chromosomal bands were resolved after 184 h of migration at 50 V. Polymorphic karyotypes were observed in different species of Fusarium , formae speciales of F. oxysporum , and races of F. oxysporum f.sp. dianthi . Using the Schizosaccharomyces pombe and Saccharomyces cerevisiae chromosomes as size standards, the size of the Fusarium genome was estimated to range from approximately 18.1 to 51.5 Mb. The suitability of electrophoretic karyotyping as a tool for strain characterization, as well as some applications in hybridization analysis of Fusarium spp., is discussed.