The growth of NH₄⁺-oxidizing bacteria and NO₂^--oxidizing bacteria and the concentration of NO₂^- were measured in mixed culture systems. The growth kinetics of the above two nitrifying bacteria in the mixed cultures were as same as those in the uni-bacterial cultures (described previously). When the NO₂^- concentration was maintained at a constant level, the growth yields of NH₄⁺- and NO₂^--oxidizing bacteria were in the ratio of about 2:1. The specific activities of NO₂^--oxidizing bacteria, calculated from the data of the uni-bacterial cultures and the mixed cultures, did not change linearly with the specific growth rates. It was suggested that energy spilling might occur at higher specific growth rates. The conditions under which the NO₂^- concentration may be maintained at a constant level were discussed.