Several closely related, xylanolytic, thermophilic bacilli were isolated from local soils on xylose-containing minimal medium. On the basis of morphology and biochemical characteristics, one of the isolates, designated strain S1812T (T = type strain), was studied further. Strain S1812T was a xylanolytic, sporulating, Gram-positive, rod-shaped bacterium. Its Gram-positive nature was confirmed by electron microscopic examination of thin sections of the cells. The isolate was a thermophilic (optimum temperature for growth, 65 degrees C), facultative anaerobe that grew on a wide range of carbon sources including glucose, lactose, starch and xylose. It expressed high levels of both xylose isomerase and xylulokinase on xylose and also on glucose. The DNA G + C content was 44 mol%. rRNA gene sequence analysis placed strain S1812T in Bacillus cluster 5; it was more closely related to Saccharococcus thermophilus than to thermophilic Bacillus species. DNA-DNA hybridization also indicated its close relationship to S. thermophilus. Based on the evidence presented, it is proposed that strain S1812T be designated Saccharococcus caldoxylosilyticus sp. nov. Strain S1812T is the type strain (= ATCC 700356T = DSM 97-987T).