Microbiological transformations of sulfur compounds discriminate to various degrees between the stable sulfur isotopes32S and34S. Comparatively little is known on isotopic effects associated with sulfur‐oxidizing organisms, and the interpretation of results is complicated since the sulfur pathways are poorly defined and compounds containing two or more sulfur atoms at different oxidation states may be involved. Dissimilatory reduction of sulfate, and sulfite reduction by certain assimilatory microorganisms, causes particularly marked isotopic effects, the expression of which depends on the extent of reaction and other incompletely defined environmental conditions. Models have been proposed to account for these effects based on current knowledge of the reduction pathways. Many of the trends observed during dissimilatory sulfate reduction in the laboratory can also be found in the modern environment leaving little doubt that microbiological factors play a significant role in determining sulfur isotope distributions in nature. However, unusually large isotopic effects, rarely approached in the laboratory, are often observed in nature. The reasons for this are not entirely clear, but in sediments it is possible that diffusional isotopic effects are imposed on biological effects.