Hypovirulence and decreased sporulation of the plant pathogenic fungus Cryphonectria (Endothia) parasitica is caused by double-stranded (ds)RNAs. These symptoms of dsRNA infection are correlated with down-regulation of at least nine major fungal polypeptides. One of the regulated polypeptides was purified to homogeneity and antibody to it was prepared. This polypeptide (cryparin) has a -glycine-serine-repeating sequence near the amino-terminal end that is typical of structural proteins and has properties of a lectin. Antibody-staining showed that this 18.6-kDa polypeptide is specific to aerial hyphae and fruiting bodies and that it accumulates in large amounts on hyphal cell surfaces. The dsRNA affects accumulation of this protein, both in the fugal hyphae and in the growth medium. Cryparin is similar in physical properties to those of the putative phytotoxin cerato-ulmin produced by the Dutch elm disease fungus. Toxicity of cryparin is not detectable, but the striking similarities between the physical properties and locations of accumulation of cryparin and cerato-ulmin in fungal fruiting structures suggest either conservation of structure or convergent evolution in function of these two proteins.