Xylanases without contaminating cellulases are produced by a wide variety of aerobic bacteria and fungi but anaerobic organisms producing such enzymes are rare. Clostridial species viz., Clostridium beijerinckii LU-1, C. acetobutylicum LU-1, C. butyricum and C. bifermentans LU-1 isolated from rumen produced xylanase (E.C. 3.2.1.8) of 1.5-4.2 U/ml on oat spelt xylan as sole carbon source. Celluloses absolutely did not support their growth. Among the carbon sources tested such as cellobiose, cellulose, filter paper cellulose (Whatman No. 1), methyl-nitrocellulose (MNC), AVICEL (microcrystalline cellulose); carboxymethylcellulose (CMC), starch and pectin did not support growth and enzyme activity. Among the xylans birch wood xylan and larch wood xylan did not support their growth. P-Glucosidase (E.C. 3.2.1.21), filter paper cellulase, carboxymethylcellulase (E.C. 3.2.1.4), methylnitrocellulase and avicelase (E.C. 3.2.1.74) were absent in the extracellular culture filtrate and no enzyme activity was found bound to the cells. In all these strains when grown on oat spelt xylan (1%, w/v), at pH 7 and 37 degrees C, the maximum enzyme production was observed within 21-24 It. In medium containing oat spell xylan, there was rapid increase in protein productivity, accumulation of extracellular endo-p-xylanase concomitantly with a decline of 65-82% of residual xylan, subsequent decrease in cell growth,,but the accumulated enzyme utilized the maximum amount of xylan in 24 It. C bifermentans LU-1 produced cell-bound P-xylosidase (E.C. 12.1.37) 0.145 U/ml. Endoxylanases had an optimum pH of 5 and temperature 50 degrees C (C. acetobutylicum LU-1 and C. butyricum LU-1) and (C. beijerinckii LU-1) 60 degrees C. Xylanases were stable between pH 4 and 8 and temperature 30-50 degrees C. Xylanase from C. bifermentans LU-1 had an optimum pH of 4 and temperature of 70 degrees C. The pH optima of 5 and good stability between 4 and 8 for the endoxylanases of C. beijerinckii LU-1,C. acetobutylicum LU-1 and C. butyricum LU-1 is suitable for processes where pH changes are common. (c) 2005 Elsevier Inc. All rights reserved.