Analysis of nucleated cell size in a minicell-producing strain of Escherichia coli and in its parental strain shows that the two distributions are considerably different. A model is proposed to account for this difference. The model states that: (i) in the mutant population, the cell poles are available as potential division sites in addition to the normally located division sites; (ii) the probability of a division occurring at any of the potential division sites is equal; and (iii) only enough “division factor” arises at each unit cell doubling to permit a single division. This factor is utilized entirely in the formation of a single septum. Thus, the occurrence of a polar division with the production of an anucleate minicell (which occurs only in the mutant strain) prevents the occurrence of a non-polar division, with the result that the average nucleated cell length is increased in minicell-producing strains. The model has been used to construct a theoretical population, and a number of parameters of the real and theoretical populations have been compared. The two populations are very similar in all of the parameters measured.