Some enzymes from extreme thermophiles and hiperthermophiles have half-lives up to 13h at ,temperatures above 100?C. They are suitable for starch and whey processing, oil mining, PCR techni and oligosaccharides synthesis. The differences in structure and function between these very stable and the prevailing less stable enzyme forms are relatively small and are comparable with those differences found among enzymes of similar stability. The relationship of both conformational stability and enzymatic activity with protein flexibility suggests that stability at temperatures far above those which are optimum for the growth of microorganism is rather unlikely.