A major goal of microbial ecology is the identification and characterization of those microorganisms which govern transformations in natural ecosystems. This review summarizes our present knowledge of microbial interactions in the natural sulfur cycle. Central to the discussion is the recent progress made in understanding the co-occurrence in natural ecosystems of sulfur bacteria with contrasting nutritional requirements and of the spatially very close associations of bacteria, the so-called phototrophic consortia (e.g. ‘Chlorochromatium aggregatum’ or ‘Pelochromatium roseum’). In a similar way, microbial interactions may also be significant during microbial transformations other than the sulfur cycle in natural ecosystems, and could also explain the low culturability of bacteria from natural samples.