By means of flow microfluorometry, the protein and nucleic acid contents of individual bacterial cells may be measured at the rate of several thousand cells per second. Accumulation of such information over a few minutes yields the composition distribution of the microbial population. These distributions have been determined at different times during batch growth of Bactillus subtilis, and the results indicate that the variance of cell composition decreases as the population passes through the exponential into the stationary phase. The relative abundance of endospores and vegetative cells as well as the protein distributions of these subpopulations may be readily determined from flow microfluorometry data. Experimental access to such details of microbial population dynamics should foster improved understanding of cell growth, spore germination, and spore formation kinetics.