Omura Bay is one of famous bays for aquicultural meaning of the pearl oyster in Japan.<BR>The bay has been often investigated on the red water by the author and some others. After several oceanographic researches, the author pointed out that Dictyocha fibula caused a red water in August 1948, and Rhizosolenia styliformis caused a white water mass after its explosive multiplication in January 1949. On these outbursts of population growth it is concluded that Dictyocha fibula and Rhizosolenia styliformis displayed respectively biotic potential, so that the former brought out the red water, the latter led to the white water. And in marginal waters, indifferent zones of O₂ were recognized, toward which fishes, crustaceans and other animals made their avoiding movements.<BR>In the case of the biological colouration of Dictyocha fibula, the O₂-lacking water mass was observed in the offshore area of the bay by the oceanographic investigation held in August 1948, as shown in text figures and, tables. (See Fig. 4 and Tab. 2)<BR>It seems that a mechanism of appearing of O₂-lacking layers near the bottom may be considered as results of biological colourations by such plankton as Dictyocha fibula and Rhizosolenia styliformis. Especially under such ecologi-cal condition as the biological colouration of Rhizosolenia styliformis, an azoic water mass was recognized owing to perished and decayed benthic communi-ties, and unpleasant odour was smelled.<BR>In summer, when the bottom-water of the bay lies stagnant and death caused by overpopulation o the plankton takes place after the explosive multiplication, there can be seen suspended, particulate organic matter swarm-ing with bacteria that produce sulphuretted hydrogen which spreads through. the water. This causes the oxygen to decrease and sometimes finally to disappear altogether. In the Bay of Omura such O₂-lacking bottom waters appear notwithstanding that the bottom is covered with organic matter and a slimy black mud is formed. So that the author terms the water which is automati-cally devoid of oxygen in consequence of explosive multiplication of the plankton and its precipitation after stagnation of that water as the azoic water mass, no matter what the sea deposits of that area may be composed of.<BR>It is likely that some biological colourations can perform a part of causing of the azoic zone of water mass in the bottom layers without regard to naturalfeature of the bottom of this bay. According to his consideration from these results, it may be said that the biological colouration in such cases as in Omura Bay affects negatively upon the process of the biological production of inlet waters.