Escherichia coli B/r cells, centrifuged to equilibrium in either self-generating or preformed gradients of Percoll, banded at an average density of 1.080 to 1.100 g/ml, depending on their growth rate and the temperature of centrifugation. Cells cultured in gradient material (70% Percoll) exhibited the same average density. At the various growth rates examined, the density of the individual cells in a steady-state population varied by less than 1% of the mean in E. coli strains B/r and B, as well as K-12. Electron microscope analysis of the density fractions of both fast- and slow-growing E. coli B/r populations suggested a small increase in density during cell constriction.