The cytochrome content was estimated in a wide range of alkaliphilic Bacillus strains with DNA G+C contents from 34.4 to 44.3%. Among the strains tested, four facultative alkaliphiles, whose G+C contents were around 36%, and one obligate alkaliphile, with a G+C content of 41.4%, possessed relatively high amounts of total cytochrome (more than 0.3 nmol.mg cells(-1)). These five alkaliphiles contained high amounts of cytochromes b and c compared with the other strains tested, but their cytochrome a contents were not noticeably different. All the tested alkaliphilic strains had much higher amounts of cytochrome than B. subtilis (neutrophile), except for two facultative alkaliphiles with G+C contents around 40%. These results suggest that various bioenergetic strategies are used by alkaliphilic Bacillus for adaptation to a high alkaline environment, and that cytochromes b and c have a bioenergetically very important role in certain kinds of alkaliphiles.