The forms and distribution of sulfur in the black bottom muds of Lake Mendota have been determined and the data combined with available background information to gain understanding of sulfur metabolism in a freshwater sulfuretum and of the generic relationships among various forms of sulfur.The suite of sulfur compounds in the lake sediments is made up of sulfide dissolved in the interstitial water, acid‐volatile sulfide, elemental sulfur, organic sulfur, and sulfates. The dominant form of sulfur in the sludge is a black, amorphous, acid‐soluble substance believed to be hydrotroilite. No pyrite or marcasite was isolated from these bottom muds. Nearly 45% of the sulfur precipitated as sulfide is apparently derived from mineralization of organic matter with the remaining fraction coming from bacterial reduction of sulfates. The sulfur analyses coupled with other evidence suggest that prior to the change in sedimentation in Lake Mendota associated with human activity the lake had developed an oxygen deficiency intense enough to cause the reduction of sulfates to sulfides but was able to maintain its basin free of black muds by processes of self‐purification.