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Interdisciplinary interactions between molecular biologists, microbial ecologists, and biochemical engineers portend weU for the biotechnological exploitation of novel, extremely thermophdic methanogens and sulfur-metabolizing archaebacteriu.[ntroduction The field of biotechnology presently relies heavily on the use of genetic engineering to make the production of a variety of biological products feasme in both a technical ind economic sense. However, there is growing interest in determining if "wild-type'' microorganisms that function under extreme conditions (i.e., high temperature, high pressure, low pH, etc.) or their products can be put directly to effective biotechnological use. The exploitation of these novel microorganisms need not preclude the use of recombinant DNA technology; on the contrary, the more we learn about the "natural" functions of bacteria from extreme environments, the greater the possibilities for genetic engineering. Among the extreme environments in which microorganisms have been found to prosper, few have generated as much scientific interest in recent years as those characterized by high temperatures [ I ] . Following reports of evidence that bacterial growth and activity may occur under deep-sea pressures at temperatures to at least 250°C [ 2 4 ] (well in excess of conventional limits) efforts by several groups of researchers [S7] to acquire more samples from submarine hydrothermal vents, hot springs, and other areas of geothermal activity in search of novel extremely thermophilic microorganisms have intensified. Though these efforts have not yet yielded confirmation of the original report [3], not quelled the controversy (pro [8] and con [9-111) that it engendered, there has been notable progress toward the isolation of new thermophiles that #Correspondence should be address to Robert M . Kelly Yody W. Deming is presently at the School of Oceanography llniversity of Washington, WB-10, Seattle, WA 98195. grow at higher and higher temperatures. In particular, Stetter and colleagues [5, 6, 12, 131 have reported and