The optical density of Chromatium cultures grown anaerobically in the light with sulfide as electron donor is mainly determined by the sulfur content of the cells. Since the sulfur content varies, growth cannot be followed in this way. However, optical density measurements are very useful to characterize the moment of sulfide depletion and maximal sulfur storage. In addition to protein or cell nitrogen, increase in structural cell material in growing cultures of Chromatium can easily be followed by bacteriochlorophyll determinations, provided that the illumination intensity is higher than the saturation intensity.