A stochastic model is used to analyze the changing size distributions of large firms in the food processing and chain store food distributing industries during 1948–1958. The model is shown to have certain predictive usefulness for anticipating changes in these distributions; however, the predictive value of the equilibrium distribution is questioned. An index of size mobility is developed from the model and the value of this index computed for each industry in each year and for the entire decade. The index clearly indicates that firms in the processing sector were characterized by greater relative size mobility than those in the distributing sector over this decade. A number of possible explanations of and deductions from the variability of the indexes, both between industries and over time, are suggested. Although no strong correlations are obtained, the descriptive and predictive value of the technique and the potential fruitfulness of further statistical work on the dynamics of industry structure are strongly indicated.