Fusarium and Giberella species were found to secrete acidic polysaccharides when grown in Czapex-Dox medium. Fusarium oxysporum showed the highest rate of secretion of acidic sugars into the growth medium. The acidic sugar-containing glycoprotein from F. oxysporum was purified from the culture filtrate to apparent homogeneity on ultracentrifugation analysis, by ammonium sulfate precipitation, Toyopearl HW-55 gel filtration and charcoal chromatography. The purified acidic sugar-containing glycoprotein has a 10% protein content and a content of 40% acidic sugars as glucuronic acid, and the neutral sugars present in this glycoprotein are mainly mannose, galactose, and glucose. The protein moiety of the glycoprotein contains a high proportion of serine and threonine residues. Treatment of the glycoprotein with alkaline solution resulted in an increase in absorbance at 241 nm, and alkaline borohydride treatment resulted in a marked decrease in the number of serine and threonine residues. We conclude that the glycoprotein has O-linked sugar chains which are mainly attached to serine and threonine residues of the protein moiety. The primary structure of the acidic polysaccharide from F. oxysporum was analyzed mainly by NMR spectrometry. The main parts of this polysaccharide have structures similar to those of uronic acid-containing polysaccharide from Fusarium sp. M7-1.