Lateral and stratigraphic variation of isotopic composition of sulfur in the Sullivan orebody has been studied as a result of extensive sampling and analytical work. A total of 221 separates were analyzed, of which 40 were galena, 24 sphalerite, 52 pyrite, and 105 pyrrhotite. Sulfur composition ranges from --10.4 to +4.7 per mil with a mean delta 34 S value of --2.2 per mil. Stratigraphic dependence of isotopic variation indicates a syngenetic origin. Changes in sulfur isotopic composition at ore-waste contacts suggest that sulfide deposition is controlled by the supply of H 2 S (which is a function largely of basin geometry) rather than metal supply. Variation in isotopic composition may be interpreted in terms of variation in the supply of H 2 S at the depositional site and a thermal gradient associated with upwelling metal-bearing solutions. The simplest model which explains the data assumes that Proterozoic sea water is the sulfur source and that basin geometry and temperature determine isotopic composition of sulfides. A number of smaller deposits in the area--Stemwinder, North Star, Kootenay King, and Quantrelle--were also sampled, resulting in 35 additional analyses. The sulfur has a range of isotopic compositions similar to that of the Sullivan, implying that sulfur source and mode of deposition of these orebodies were similar. Optimum conditions for ore accumulation occurred more than once and at several sites in the area during middle Proterozoic time.