A defined medium deficient in both niacin and biotin supported neither Proteus vulgaris nor Bacillus polymyxa in pure culture, but a mixed culture grew well. Presumably each can supply a vitamin requirement for the other. Continuous flow cultivation of mixed cultures exhibited oscillatory population levels of each species. P. vulgaris produced an inhibitor which accumulated to inhibit growth of B. polymyxa, and the vitamin interdependency caused the former to also cease growing. After continued pumping diluted the inhibitor, growth resumed. The inhibitor seems to be a protein, because proteolytic enzymes added to the culture gave higher, non-oscillatory population levels.