ABSTRACTIsolated hyphal walls of the OomyceteLagenidium giganteumwere studied by cytochemical, biochemical, and x-ray diffraction analyses. The principal wall component was (β-glucan, accounting for 77% of wall dry weight, the major portion of which is composed of (1→3)- and (1→6)-linked glucosyl residues. The qualitative presence of cellulose (β-1→4 glucan) was demonstrated cytochemically and by x-ray diffraction analysis. Cellulose accounts for 9.1% of total wall dry weight. Hyphal walls also contain mannose, protein, ash (inorganic), and lipid components, accounting for 4.0, 5.8, 1.4, and 2.5 percent of wall dry weight, respectively. Trace quantities (<1%) of glucosamine were detected in wall hydroly-sates, but there is no cytochemical evidence for the presence of chitin in hyphal walls. In general, the hyphal wall composition ofL. giganteumis similar to that observed in most other Oomycetes.