An alkalothermophilic Thermomonospora sp. producing high levels of xylanase was isolated from self-heating compost. The culture produced 125 IU/ml of xylanase when grown in shake flasks at pH 9 and 50 degreesC for 96 h. The culture filtrate also contained cellulase (23 IU/ml), mannanase(1 IU/ml) and beta -xylosidase (0.1 IU/ml) activities. The xylanase was active at a broad range of pH (5-9) and temperature (40-90 degreesC). The optimum pH and temperature were 7 and 70 degreesC, respectively. The enzyme was stable in the pH range 5-8 and was thermostable with half-lives of 8 and 4 h at 60 degreesC and 70 degreesC, respectively, but only 9 min at 80 degreesC. The effects of a variety of compounds to enhance the stability of xylanase at 80 degreesC was studied. Addition of sorbitol, mannitol and glycerol increased the thermostability of xylanase in proportion to the number of hydroxyl groups per polyol molecule. Glycine also offered protection against thermoinactivation, Xylan, trehalose, gelatin and trehalose-gelatin mixture had marginal effect on the thermostability of xylanase at 80 degreesC.