This study was part of a larger research programme designed to obtain a method of improving the feed value of waste straw to ruminants, using a selected microorganism. The work reported here was carried out to assess the potential of Coprinus cinereus (Schaeff ex Fr.) S.F. Gray sensu Konr. for this purpose, to upscale the most promising treatments and to investigate the natural colonisation of straw by the fungus. The breakdown of sterile barley straw was investigated over a range of pHs maintained with ammonia and hydrochloric acid solutions. Hemicellulose and cellulose were found to be attacked, with hemicellulose decomposed preferentially. At optimum growth conditions, in vitro digestibility of the straw was increased to the level of good quality hay, although this declined after peaking. Digestibility increase was correlated with utilisation of hemicellulose and cellulose in the short term. Larger scale biodegradation trials using unsterile straw showed that a consistently upgraded product could be obtained using an inoculum of C. cinereus spores in 1% wjw ammonia solution with loose straw in polythene sacks. The low pro­tein content of the upgraded material indicated that it was unsuitable as a complete ruminant diet. A technique was developed to study the activity of C. cinereus and other fungi in cereal field soil. It was shown that C. cinereus is an active component of the fungus flora of barley field soil, indicating that the propagules of C. cinereus found in harvested straw arise from the soil. The feasibility of linking a straw biodegradation process to an animal slurry treatment system, which serves as an ammonia source was investigated. The economic aspects of such a scheme were discussed.