The physiology of ammonia assimilation enzymes was examined in Bacillus sp. FE-1, a thermophilic marine bacterium. Glutamine synthetase (GS) and glutamate synthase (GOGAT) activities varied with the nitrogen source present in the medium, ranging as much as 10-fold for the former and 2.5-fold for the latter. Glutamate dehydrogenase (GDH) was detected but, under the growth conditions studied, levels were not affected by the nitrogen source. Anaerobic growth in the presence of nitrate yielded enzyme levels that were not significantly different from those measured under aerobic growth. Partially purified GS exhibited a temperature optimum between 65-degrees and 75-degrees-C. The enzyme's Mn2+-dependent reverse transferase activity was stimulated by K2SO4 and demonstrated some tolerance to NaCl. Hyperbolic kinetics were observed for ammonium, with an apparent K(M) of 1.0 mM.