In the sea as a whole the combined activities of bacteria and allied microorganisms tend to create environmental conditions which are conductive to the growth of plants and animals. However, in localized regions certain types of microbiological activities may be inimical to the well-being of other organisms.<br> An outstanding example of the antibiotic activi ?? ies of bacteria in the sea is the production of sulfide in quantities which are toxic to both animals and plants.<br> It is already known that a considerable amount of sulfides often occurred in stagnant bodies of coastal waters which received a large amount of industrial or municipal drainage.<br> The sulfides occurring in the sea may partly result from the anaerobic decomposition of albuminous compounds. More is believed to result from the bacterial reduction of sulfate.<br> Although a few papers concerning the determination of sulfides which occurred in the sea have been published, it appears that few workers have studied the occurrence and the activities of sulfate-reducing bacteria in the sea in relation to the pollution of coastal waters up to the present.<br> Such being the case, the present series of study were undertaken with the aim of elucidating the factors which participate in the sulfide production by bacteria in the sea, with special reference to the influence of the inflow of pulp mill drainage upon the sulfide production in the coastal regions of the sea.<br> The present paper embodies our ecological observations carried out in Hiroshima Bay. Many samples of sea water and bottom mud were collected from different depth at various stations in Hiroshima Bay (Fig. 1), and immediately the number of sulfate-reducing bacteria was counted by means of the extinction dilution method. In addition to the bacterioloigcal examination, the chlorinities of water samples and the amounts of C. O. D., ignition loss, total sulfides and free H₂S of mud samples were determined. And also the temperatures of all samples were recorded.<br> Results obtained are as shown in Tables 1, 2 and 3 and may be summarized as follows:<br> 1) Relatively small number of sulfate-reducing bacteria were evenly distributed in the sea water regardless of the depth of the water, the distance from land or the amount of terrigenous pollution.<br> 2) Larger number of sulfate-reducing bacteria were found in bottom mud at nearly all stations examined. The number of sulfate-reducing bacteria contained in bottom mud was correlated with the sulfide content of the mud as shown in Figs. 2 and 4.<br> The result suggests that the greater part of the sulfides contained in the mud were produced by the sulfate-reducing bacteria.<br> The distribution of sulfate-reducing bacteria in bottom mud or the sulfide content of bottom mud was directly correlated with the organic content (Figs. 3 and 5), but not correlated with the distance from land or the depth of the overlying water. As shown in Fig. 6 the population of sulfate-reducing bacteria in marine sediment was presumably more largely dependent upon the quality-decomposability-of the organic matter than upon the total quantity of organic matter present.<br> It is thought that a large amount of easily utilizable organic matter brought onto the bottom mud by the industrial drainage, such as pulp mill waste, may vigorously accelerate the activity of sulfate-reducing bacteria present in the bottom mud of the coastal regions of the sea.