Chromosomal DNAs from nine strains of Fusarium oxysporum formae speciales were analysed by pulsed field gel electrophoresis (PFGE). Intact chromosomal DNA were prepared from fungal protoplasts. About 7-13 chromosomal DNA bands were separated in nine Fusarium oxysporum formaespeciales, ranging in size from 0.8 to 6.7 Mb. Considering that some of the chromosomal bands represented unresolved doublets, the total genome size was estimated to vary between at least 22.3 and 45.5 Mb. Gels run under different conditions were Southern blotted and probed with randomly selected genomic DNA from Fusarium oxysporum f. sp. lilii. Electrophoretix karyotyping revealed some differences within and between Fusarium oxysporum formae speciales.