The theoretical distributions of cell masses in exponential cultures of bacteria were derived for both total cells and cells having formed a constriction in preparation for division. The parameters used for this derivation include the mass doubling time, τ, the T-period, and 3 statistical parameters (h, σ1, σ2) which describe the variability of the cell cycle. The theoretical distributions were compared with observed distributions from E. coli B/rA growing in glucose minimal medium (45 min doubling time) to determine whether a stochastic process in the division pathway affects the time of initiation of constriction or the duration of the constriction process. The results indicate that the stochastic process determines the onset rather than the completion of constriction and that the timing of this process is coupled (6% variability, = σ1) to a given cell mass. The values obtained for the duration of the T-period, T = 9·3 min, and for a half-life parameter associated with the stochastic process, h = 4·3 min, agree with previously reported data.