Exoglucanase, a β-1, 4-glucan cellobiohydrolase with specific affinity for β-1, 4-linked glucans, and wheat germ agglutinin, a lectin with N-acetylglucosamine-binding specifity, were used for localizing cellulosic β-1, 4-glucans and chitin in the cell walls of six plant pathogenic fungi. Both chitin and cellulose were found to occur in the cell walls of the oomycete fungus Pythium ultimum whereas only cellulose was present in those of Phythophthora parasitica var. nicotianae, another oomycete fungus. The two compounds were also simultaneously detected in the cell walls of two ascomycetes, Ophiostoma ulmi and Colletotrichum lindemuthianum. Finally, only chitin could be detected in the cell walls of the ascomycete Fusarium oxysporum f.sp. radicis-lycopersici (FORL) and the basidiomycete Rhizoctonia solani. The dual occurrence of chitin and cellulose in Pythium ultimum cell walls was further confirmed by enzymatic extraction performed on isolated cell walls. Treatment of Pythium ultimum cell walls with hen egg white lysozyme, an enzyme with strong chitinolitic activity, prior to labeling with wheat germ agglutinin - ovomucoid - gold complex, resulted in a near abolition of labeling. Similar results were obtained with FORL cell walls used as positive controls to assess the validity of the enzymatic extraction process. When cell walls of Pythium ultimum were treated with a mixture of hen egg white lysozyme and cellulase, both chitin and cellulose were hydrolysed as shown by the considerable reduction of labeling. Thus, these data provide evidence for the presence of chitin in Pythium ultimum cell walls and suggest that classification of Oomycetes as a cellulose-glucan group may be reconsidered.Key words: Pythium ultimum, chitin, cellulose.