Using the most effective rapid method for the detection of restriction endonucleases (ENases) in microorganisms, 32 thermophilic producers have been isolated. All strains belong to the genus Bacillus. Thermostable isoschizomers of ENases, such as AvaI, BbvI, BbvII, BclI, BsaBI, BsiYI, BsrI, BstEII, BstNI, Cfr10I, ClaI, FspI, HaeIII, HpaII, Ksp632I and SfeI, were isolated.