Abstract: A feed additive that provides increased feed conversion ratio is prepared containing a physiologically acceptable carrier and one or more of five thermostable xylanases from Microtetraspora flexuosa, preferably strain ATCC 35864. The xylanases can tolerate heating for one minute in a water bath at 95.degree. C. without substantial loss of activity, and tolerate alkaline conditions in a pH range of 7.0-10.0. A feed for poultry or pigs contains the xylanases in a feed containing at least 20% by weight cereal. The five xylanases have the following characteristics: (i) a molecular weight of about 33,100 daltons, a pI of about 8.5, and a maximum activity at pH 7.0-7.5 at a temperature of about 70.degree. C.; (ii) a molecular weight of about 33,300 daltons, a pI of about 7.5, and a maximum activity at pH 7.0-7.5 at a temperature of about 65.degree. C.; (iii) a molecular weight of about 31,000 daltons, a pI of about 6.2, and a maximum activity at pH 7.5 at a temperature of about 65.degree. C.