1. The isolation and characterization of a restriction endonuclease from a thermophilic strain of Bacillus is described. 2. The enzyme recognizes the palindromic sequence 5'...GGCC...3' as determined by PEI-cellulose chromatography of pancreatic DNAse and snake venom phosphodiesterase digestion products of labelled DNA fragments, analysis of restriction digests and direct sequence analysis. 3. The enzyme, denominated BspBR, is an isoschizomer of HaeIII and BspRI.