SUMMARYIsolated hyphal walls of the oomycete,Leptomitus lacteus, contain approximately 74%β-glucans, the major portion of which is apparently composed of (1→3)- and (1→6)-linked glucosyl residues. Cellulose occurs as cellulose I of relatively low crystallinity and constitutes approximately 6.2–7.5% of wall weight. Hexosamine polymers account for 14.5% of wall weight. The major portion of the hexosamine fraction (13.8%) is insoluble and assumed to be present as chitin. The qualitative presence of chitin was demonstrated by X-ray diffraction analysis. The protein content of 3.6% is made up of a broad assortment of the common amino acids plus low levels of hydroxyproline. Additionally, small amounts of readily extracted and bound lipids (1.8% total) and ash (0.3%) are present. In general, the wall composition ofL. lacteusis quite similar to that of the related fungus,Apodachlyasp. and this work supports the assertion that walls of leptomitaceous species are chitinous in contrast with walls of rhipidiaceous species which apparently are not.