Average specific density of individual cells of pure cultures of Chromatium warmingii and Chromatium vinosum were measured by isopicnic gradient centrifugation with Percoll during growth at constant illumination as a function of the increasing content of intracellular sulfur. Cell number and volume, bacteriochlorophyll a, sulfide, and sulfur were followed in the cultures along with cellular buoyant density. Poly-β-hydroxybutyrate was monitored at several points during growth of the cultures. The density of C. warmingii changed from 1.071 to 1.108 g cm-3 (sulfur content per cell varied from 0 to 1.71pg). C. vinosum changed its density from 1.096 to 1.160 g cm-3 (sulfur content per cell varied from 0 to 0.43 pg). Maximum sulfur content in pg of sulfur per μm3 of cell volume were 0.178 for C. warmingii and 0.294 for C. vinosum. Measurement of the differences in buoyant density, volume and sulfur content before and after ethanol extraction of cells with and without intracellular sulfur, allowed tentatively to estimate the density of sulfur inside the cells as 1.219 g cm-3. Isolation of sulfur globules and centrifugation in density gradients gave a density higher than 1.143 g cm-3 for these intracellular inclusions.