In the Sea of Harima, the situation of proliferation of Chattonella is fluctuating with the period of about 5 years. The mechanism of this fluctuating is expressed to this report. Certain bacteria promoted the proliferation of Chattonella. And other bacteria inhibitted the proliferation. The situation of proliferation of Chattonella was controlled by these bacteria. The bacteria that parasitizes Chattonella antiqua and muders it was isolated. This bacteria was able to utilize C. antiqua as the nutrients for proliferation.