Interaction of the populations of two species of blue-green algae Tolypothrix tenuis and Anabaena variabilis "1058", in mixed culture, was observed in three different ways: (1) the growth competition was studied in mixed-culture, where the two species grew in one batch of culture. (2) The effect of the filtrate, the extracellular products, from the culture of one species on the growth of the other species was studied. (3) In filtrated culture, two algal species were cultivated separately in either side a U-form container partitioned by a micro-pore membrane in the middle. The extracellular products were permeable through the membrane from one side to the other side. The influence of the biologically active substances produced by the algae at different growth stages can be observed and estimated. The growth was measured by dry weight of biomass and chlorophyll-a content. The proportion of components of phycobiliprotein (e.g. ratio of phycocyanin to phycoerythrin) was estimated as the specific growth rate of two blue-greens in the mixed cultures. 1. The results obtained are summarized as follows: There were three types of the bioactivity of the extracellular products: (1) The lethal effects on each other caused th by the lethal agents of these two blue-greens were different. (2) The suppressive effects on growth by each other in one community were also found. (3) Effect on growth promotion exhibited only in the extracellular products of A. variabilis "1058". 2. The results indicate that there occur a direct competitive interaction of two populations. This competition was controlled by the bio-active substances of the extracellular products which might regulate the structural composition and in turn, the succession of the community.