Mechanically isolated hyphal walls of the oomyceteApodachlyella completa (Humphrey) Indoh. (Leptomitales) were investigated by means of biochemical, cytochemical, and X-ray analyses. Microscopic examination of the cell wall preparations revealed them to be free of cytoplasmic contaminants β-Glucans account for 88% of wall weight and contain predominantly I→3 and I→6 linkages. Cellulose was demonstrated cytochemically and by X-ray diffraction and accounts for approximately 6% of wall dry weight. Additionally, walls contain 5% insoluble hexosamine, 4.8% protein, 1.1% lipid, and 0.5% ash. Chitin was qualitatively detected by means of cytochemical and X-ray analysis. The cell wall composition ofA. completa is similar to that reported for other oomycetes, in that β-glucans are the primary wall constituents. However, this fungus shows the unusual feature of chitin occurring simultaneously with cellulose, as do the closely related speciesApodachlya sp. andLeptomitus lacteus. *** DIRECT SUPPORT *** A01R4022 00007