Abstract: The high Sulphide Content (SC) of mud is found to be one of the most important causes for unfavorable geo-environmental condition for the benthos in the acid contaminated Ariake Sea, Japan. In order to identify the temporal variation of the sulphide content of the mud, field tests were conducted in the Iida and Higashiyoka tidal flat mud area of the Ariake Sea during the period 2002-2005. The collected column of mud was cut into a number of specific layers the thickness of which increased with increasing depth. The sulphide content of the mud in each layer was measured along with other physico-chemical properties. The sulphide content of the mud varies from 0.5 to 2.0x10-3 kg kg-1 dry-mud, which is above the allowable limit (0.2x10-3 kg kg-1 dry-mud) for the favorable condition of the benthos` life. In summer, the peak value of the sulphide content varies from 1.5 to 2.0x10-3 kg kg-1 dry-mud since the bacterial activities increase with the elevated seasonal temperature. The peak value of the sulphide content, on the other hand, is relatively low (0.5-1.0x10-3 kg kg-1 dry-mud) during the winter as the activities of bacteria as well as bioturbation decrease in the cold temperature. The maximum sulphide content depth was also influenced by the seasonal temperature. The highest sulphide content in the mud that decreases gradually from 2.0x10-3 kg kg-1 dry-mud at 2002 to 1.3x10-3 kg kg-1 dry-mud at 2005 indicates growing favorable conditions for the growth of benthos.