The behavior of cell buoyant density during the cell cycle has been determined for a number of different cell types, including bacteria, yeast, and mammalian cells. Mean buoyant density was extremely constant and independent of cell age during the cell cycle of the bacterium Escherichia coli, the fission yeast Schizosaccharomyces cerevisiae, the protozoan Amoebae proteus, cells from suspension cell cultures of mouse lymphoma and myeloma, and Chinese hamster ovary cells. In all of these cases, the buoyant densities of these cells were very narrowly distributed, with coefficients of variation of 0.1 to 0.3%. In contrast, buoyant density was variable in cells with thick cell walls and high buoyant densities. Density varied markedly during the cell cycle of the budding yeast Schizosaccharomyces cerevisiae and of the bacterium Streptococcus faecium. The average buoyant densities of cells in exponentially growing cultures of E. coli or Schizosaccharomyces pombe were also independent of growth rate of the cultures. Experiments with E. coli have established that cell buoyant density is controlled by the osmoregulatory system. Although the regulatory mechanisms for this control are unknown, the results suggest that the same or similar mechanisms regulate buoyant density in all of the cells that do not have unduly heavy cell walls and, therefore, these regulatory mechanisms were either conserved during evolution or reflect the convergent evolution found for organic osmolytes.