Alkaliphilic microorganisms are widely distributed in nature. Among them, several aerobic alkaliphilic Bacillus spp. have been studied in terms of their mechanisms of physiological adaptation under an extremely alkaline condition. On the basis of chemiosmotic theories, neutrophiles produce H+ electrochemical potential (Deltap), which is the sum of transmembrane pH gradient (DeltapH) (alkaline, inside) and membrane potential (Deltapsi) (negative, inside), for active transport of solutes, motility, and ATP synthesis. In the case of alkaliphiles, it seems that Mitchell's chemiosmotic theories alone cannot explain clearly their positive H+ electrochemical potential (Deltap) across the membrane because these bacteria exhibit DeltapH in a direction opposite to that in neutrophiles, which seems to be causing extensively negative to produce energy, theoretically. Nevertheless, it is reported that ATP synthesis is more rapid at high alkaline pH than at near neutral pH in the facultative alkaliphile Bacillus pseudofirmus OF4. The respiratory system of alkaliphilic microorganisms might have an important role in compensating the reversed transmembrane pH gradient by means of ATP synthesis. To understand the function of the respiratory system in alkaliphiles, several respiratory components in alkaliphilic Bacillus spp. were isolated and characterized. In these studies, respiratory components of alkaliphiles exhibiting several unique characteristics are identified. These characteristics may have an important role in obtaining energy in alkaline environments. Information obtained from bioenergetics studies of alkaliphiles will reveal new important findings on general energy coupling phenomena.