Organic and inorganic sulfur constituents in streams, the water column, seston, and sediments of an oligotrophic Adirondack lake were measured for 2 years (1981)(1982)(1983). Soluble organic S constituents (C-bonded S and ester sulfate) were l-18% of total S in streams, the water column, and lake outlet. Seston S (0.3-1.2% dry mass) in South Lake consisted of ester sulfate (44-59%), C-bonded S (32-43%), sulfate (10-l 6%), and nonsulfate inorganic S (~2%). Rates of S deposition measured in sediment traps were highest after spring turnover. The organic matter content (52-81% dry mass) of traps at 5, 8, and 15.5 m showed no significant differences.Net mineralization of seston inputs was 26% based on mass balance calculations, with 43% of the ester sulfate input mineralized. Because most ofthe S input to the sediments was not mineralized, organic S accumulated and constituted the major (74% of total S) S component of the sediment.Most studies of sulfur cycling in freshwater systems have focused on the dynamics of inorganic sulfate and sulfide for which transformations are regulated by redox reactions (Stuiver 1967; Berner 197 1; Cook 198 1). Organic S in freshwater lake sediments has been less studied. Hesse (1958) found that >90% of the total S in the sediments of Lake Victoria was in organic form. Nriagu (1968) found that organic S composed most of the total S in marl deposits and a small amount of total S in sludge sediments in Lake Mendota. Mitchell et al. (198 1,1984) have shown that organic S usually constituted >80% of the total S in the sediments of three New York lakes. King and Klug (1982) found in hypereutrophic Wintergreen Lake sediments, where low redox conditions and high carbon levels would seem to favor pyrite formation, that organic S was still >80% of total S and that pyrite was < 10% oftotal S. Altschuler et al. (1983) found organic S predominant in peat deposits across the Everglades basin and indicated that pyritic S formation in this reduced substrate was dependent on the use of organic S compounds in dissimilatory respiration by S-reducing bacteria. Mitchell et al. (1984) and Landers and Mitchell (unpubl. data) indicated that sed-1 Present address: Department of Forestry, Univ. Illinois, Urbana 6 180 1.iment S constituents and transformations show considerable temporal changes and that 35S as sulfate was rapidly converted into organic forms. For South Lake, on average 50 and 12% of the added 35S was transformed into ester sulfate and refractory S (C-bonded S and pyritic S) and 36% remained as sulfate (Landers and Mitchell unpubl. data). Less than 2% of the 35S was found as reduced, volatile inorganic S in their sediment core experiments. The uptake of 35S as sulfate was also shown to have significant temporal changes and differences among study lakes (Landers and Mitchell unpubl. data).Seston is the major contributor of material to sediments in most lakes. King and Klug (1982) found that seston S was mostly ester sulfate and C-bonded S. This may indicate that sedimented organic S is...