Accumulation of chitinous material in Agaricus bisporus stalks was determined during postharvest storage at 4 and 25 degrees C. The chitinous material was extracted after alkali treatment and acid reflux of alkali insoluble material and analyzed for yield, purity, degree of acetylation (DA), and crystallinity. The total glucosamine content in mushroom stalks increased from 7.14% dry weight (DW) at harvest (day 0) to 11.00% DW and 19.02% DW after 15 days of storage at 4 degrees C and 5 days of storage at 25 degrees C, respectively. The yield of crude chitin isolated from stalks stored at 25 degrees C for 5 days was 27.00% DW and consisted of 46.08% glucosamine and 20.94% neutral polysaccharides. The DA of fungal chitin was from 75.8 to 87.6%, which is similar to commercially available crustacean chitin. The yield of crude fungal chitin of 0.65-1.15% on a fresh basis indicates the potential for the utilization of these mushroom byproducts.