Some hot springs located in the west of Turkey were investigated regarding the presence of xylanolytic thermophilic microorganisms. Based on phenotyping characteristics and 16S rRNA gene sequence analysis, 9 of these xylanolytic isolates belonged to the genus Anoxybacillus, and grew optimally at about 50-60 degrees C on nutrient agar. The 16S rRNA gene sequence analysis showed that these isolates resembled the Anoxybacillus species >= 97 and these isolates are members of the genus Anoxybacillus. Based on the DNA-DNA hybridization study, SDS-PAGE profile, and biochemical and physiological features, 13, CT1Sari, and BT2.1 isolates are new strains of Anoxybacillus gonensis; 14.2 and B9.3 isolates are new strains of A. voinovskiensis; and 14.1, AC26, ACT14, and ACT2Sari isolates are new strains of A. kestanbolensis. The presence of xylanase activities, their optimum temperature, pH stability, and optimum pH were also investigated. The isolates ACT2Sari and ACT14 had the highest temperature optima (75 degrees C), and 13, CT1Sari, BT2.1, ACT2Sari, and ACT14 had the highest pH optima (pH 9.0) of xylanase. The xylanases of 13, CT1Sari, BT2.1, 14.1, ACT2Sari, AC26, and ACT14 were optimally active both at alkaline pH and elevated temperature, and xylanases of 14.1, ACT2Sari, AC26, and ACT 14 also were stable at alkaline pH.