Cell buoyant densities were determined in exponentially growing cultures of Escherichia coli B/r NC32 and E. coli K-12 PAT84 by equilibrium centrifugation in Percoll gradients. Distributions within density bands were measured as viable cells or total numbers of cells. At all growth rates, buoyant densities had narrow normal distributions with essentially the same value for the coefficient of variation, 0.15%. When the density distributions were determined in Ficoll gradients, they were more than twice as broad, but this increased variability was associated with the binding of Ficoll to the bacteria. Mean cell volumes and cell lengths were independent of cell densities in Percoll bands, within experimental errors, both in slowly and in rapidly growing cultures. Buoyant densities of cells separated by size, and therefore by age, in sucrose gradients also were observed to be independent of age. The results make unlikely any stepwise change in mean buoyant density of 0.1% or more during the cycle. These results also make it unlikely that signaling functions for cell division or for other cell cycle events are provided by density variations.