Recently, in Japan, Vibrio parahaemolyticus has been considered to be a very important role as a causative agent of food poisoning from eating raw fish and shellfish in summer season. Several studies on the distribution of the bacteria in natural environments revealed that the organisms are marine inhabitants. Although the organisms are divided into three biotypes, 1, 2 and 3, it is believed that the organisms causative of the disease are Type 1 alone. In the present study, viabilities of the three types of organism in various contaminated sea water being added with sewage or peptone water were compared at 25° and 14°C. Results obtained were as follows: Firstly, the persistency of the organisms in raw sewage samples were investigated. Only within one hour, all of the Type 3 organisms and more than 99 per cent of the Types 1 and 2 disappeared (Fig. 1). In the several mixtures of various ratios of sea water and sewage, the Type 3 organisms were died within 2 days either at 25° or 14°C, while Types 1 and 2 survived later on. In the mixtures containing 50 per cent or more of the sea water, the Type 2 organisms shown much higher viability than those of Type 1 either at 25° or at 14°C, however in the case of 10 per cent sea water kept at 25°C, an inverse result was obtained (Fig. 2A). Furthermore, similar experiments were conducted in which 300ppm peptone water was used instead of sewage. As shown in Table 3, five test groups of combination consisting of the 3 types were prepared and incubated only at 25°C (Fig. 3). In the cases of 10 per cent sea water, the survivals of Type 1 organisms were invariably greater than those of Type 2. In addition, in the mixture of 50 per cent sea water, two strains of Type 1 of marine origin (Groups 4 and 5) multiplied markedly within 2 days, while the other 3 strains of patient origin did not. These facts aforementioned may suggest that the numerical interrelationships among the populations of the 3 types in natural sea water are greatly influenced by land drainage.