Three cultivars of soft wheat were cultivated with or without a growth regulator, Terpal(R) (BASF), and the differences in straw chemical compositions and biodegradabilities were determined. Significant differences between cultivars were found Although no pattern of growth-regulator treatment effects could be drawn, differences between treated and non-treated samples were observed for each cultivar. Correlations between the biodegradability of straw cell-wall, estimated with polysaccharidases or cultures of Scytalidium thermophilum, Pleurotus ostreatus, Lentinula edodes, and lignin or nitrogen contents of straw were significantly positive. Correlations between biodegradability and ash contents were negative. Estimations of straw quality both by chemical analyses and proper measurements of biodegradability could be useful for mushroom production.