Fukuoka Bay can be divided into three main parts, viz., entrance of Fukuoka Bay, Imazu Bay and Hakata Bay. During February, May, and November, 1970, the bottom mud samples were collected from Fukuoka Bay, especially from Hakata Bay. Pigments content, ignition loss (%), and C/N value of these bottom muds were measured, and were deduced the distribution of organic matter from these results and discussed their characteristics in the water area where red tides occurred frequently. 1) About 90-100% of pigments contained in bottom mud were pheo-pigments, and distribution pattern of pigments content in each month were associated with the current in the water area. 2) Ignition loss (%) at 800℃ took a larger value as much as 60-120% in excess of that at 500℃. Distribution pattern of ignition loss (%) corresponded to that of pigments content. 3) Distribution pattern of C/N value was in relation to that of pigments content. 4) Basing on the special characters of bottom mud deduced from the relationship between ignition loss (%) and C/N value, it is possible to divide Fukuoka Bay into four areas, viz., a) Imazu Bay and sea route, b) West-Hakata Bay, c) East-Hakata Bay and the inside of harbor, and d) area with the mud of a plenty of organic matter content in East-Hakata Bay. 5) The water area where red tides occur frequently coincides with the above c) and d), where a plenty of organic matter content is in mud.