The structure of polygalactosamine purified from the culture fluid of Aspergillus parasiticus AHU 7165 was studied. Partial acid hydrolysis of this polysaccharide, in which 55 to 65% of the monosaccharide residues were N-unsubstituted, gave a series of galactosamine oligosaccharides (dimer to hexamer) in a good yield. From the data on analyses of the polysaccharide and its oligosaccharides by gel filtration, periodate oxidation, methylation, and proton NMR measurement, the polysaccharide was characterized as a linear chain of alpha (1-4)-linked galactosamine residues. The N-unsubstituted galactosamine residues are probably distributed in a random fashion over the polysaccharide chain.