In order to elucidate the mechanism of fungal heterokayosis, a wild type strain of Fusarium oxysporum f. sp. vasinfectum was isolated from the cotton field in Anyang, Henan Province. Through single hyphal-tip isolation, a heterokaryon, Ag149, was obtained, and its two different phenotypic segregants, Ag149-I and Ag149-III, were separated from the mutated sectors on colony of the heterokaryon. They have remarkable differences on color of colony, morphology of hypha and pathogenicity. After analyzing by RAPD on their nuclear DNA with 100 random primers, no polymorphic difference was found among them. On going to find the different expressed gene, the mRNA differential display method was performed. Two kinds of reverse transcriptase AMV and MMLV, and a kit which consists of three kinds of 3' terminal anchor primers and eight kinds of 5' terminal arbitrary primers were used in differential display PCR (DD-PCR). Total RNA as template was reverse transcribed into corresponding cDNA by 3' terminal anchor primers, and the cDNA were amplified by polymerase chain reaction with a set of one same 3' anchor primer and one 5' arbitrary primer. The PCR products were then resolved on denaturing polyacylamimide gel, and the cDNA bands were visualized by silver staining. Among the 144 PCR products, 19 differentially expressed cDNA fragments ranged from 300 bp to 700 bp were purified. All of them were ligated to pGEM-T vector respectively for sequencing and Rev-Northern blotting. Two cDNA fragments (G5 and C6) were observed to be positive after Rev-Northern blotting. The C6 was highly expressed in the heterokaryon Ag149 and its segregant Ag149-I. It is 564 bp in length and can be predicted 77 amino acids from the beginning of the 3rd to 233th base, and then searched from GenBank. The amino acid sequence of C6 shared homologies with the 6th subunit of NADH dehydrogenase found in some bacteria, plants and animals at 30% - 70% level. While the G5 was highly expressed in Ag149 and its segregant Ag149-III. It is 432 bp in length and can be predicted 101 amino acids from the beginning of the 2nd to 304th base, and the amino acid sequence is 35% homologies comparing with the tetracycline efflux protein (OtrB) of Streptomyces rimosus. We also checked these two kinds of the pGEM-T vector harboring cDNA fragments (C6 and G5) by Southern blotting with their nuclear DNA and mitochondrial DNA (mtDNA) separately. The positive identification signals only appeared from nuclear DNA,and it addressed that C6 and G5 locate on their nuclear genome. The result indicated that the difference between the heterokaryon and its segregants is distinct on gene transcriptional level. Thus a molecular evidence for the formation of heterokaryon in filamentous fungi was provided.