The employment of thermophiles and their enzymes is effective for the degradation of hard-to-degrade proteins. In this report, we describe the efficient degradation of fibroin, one major protein of silk fiber, by a thermophile and its enzymes. Brevibacillus thermoruber YAS-1, a thermophile which produces a proteinase with high action on fibroin fiber, was screened from samples obtained at the Manza hot springs, Gunma, Japan. The time course of fibroin degradation by the strain was investigated with a culture supernatant or concentrated enzyme sample. The breakdown of fibroin molecules was confirmed by the use of scanning electron microscopy, and the fibroin solubilized by the enzyme action was examined by amino acid composition analyses together with SDS-PAGE analysis. Herein, we demonstrate the significant degradation of fibroin fiber by strain YAS-1 and the potentials of its fibroinolytic enzyme(s). (C) 2009, The Society for Biotechnology. Japan. All rights reserved.