The present invention is directed to thermostable xylanase enzymes are suitable for feed pelleting applications. The novel xylanase enzymes comprise at least 40% of their optimal activity from a pH range from about pH 3.5 to about pH 6.0, and from about 40 to about 60.degree. C., and exhibit at least 30% of their optimal activity after a pre-incubation step for 30 minutes at 70.degree. C. in the presence of 40% glycerol. Also disclosed are modified xylanase molecules comprising either a basic amino acid at position 162 (TrX numbering), or its equivalent position in other xylanase molecules, at least one disulfide bridge, or a combination thereof. The thermostable xylanase molecules of the present invention have a physiological temperature and pH optima and are useful as animal feeds additives since they can withstand the heat associated with feed sterilization and pellet formation, yet they exhibit optimal activity within an animal to aid in breakdown of ingested feed.