The source of free sulfide in tidal flat sediments and the mechanisms leading to its generation were investigated by growing mixed cultures of anaerobic sediment bacteria in media containing organic material extracted from sediments and benthic algae. Sulfide production was observed in all cultures; stoichiometric considerations indicated sulfate reduction was the most important mechanism. Sulfide production rates were directly dependent on the transient concentrations of both sulfate and soluble organic carbon in the media. The tidal flat deposits of estuaries are areas of extreme complexity and activity (Bass Becking and Wood 1955; Wood 1965). They serve as both sources and sinks for a large variety of compounds and materials, produced by processes equally many and varied, and are in a continual state of exchange with the surrounding water and air. Estuaries, and tidal flats in particular, have been significantly affected by land filling, diking, dredging, and other forms of pollution. It is imperative that the ecology of these regions be understood so that rational decisions can be made regarding their use and management. A model of the tidal flat system is shown in Fig. 1, at a level of resolution chosen to illustrate those components and processes relevant to this study; omission of many obvious factors does not imply they are unimportant in the ecology of tidal flat areas. A mathematical model of free sulfide release from a benthic system has been developed (Bella et al. 1972). It assumes a source of free sulfide within the anaerobic zone of the sediment and describes its diffusion and oxidation within the aerobic zone. The magnitude and rate of replenishment of this assumed source of free sulfide and its relationship to conditions within the deposit is important to an understanding of the phenomenon of sulfide release and of the tidal flat system. Berner (1972) has also stressed the need for an understanding of