Escherichia coli and Azotobacter agilis were grown in minimal media until a steady state was established. The distribution of cell size was determined electronically. From the equation of Collins and Richmond, the growth rate of individual cells was computed as a function of size. The main features of the growth of individual E. coli and A. agilis cells revealed by this work were: the specific growth rate decreased at the time of division, and both the absolute and specific growth rates increased between divisions. The frequency function of interdivision times was computed and was found to be positively skewed with a coefficient of variation of approximately 0.3. The results supported the hypothesis of Koch and Schaechter that the size of an individual cell at division is highly regulated.