This study has four objectives: (1) to describe the dynamics of growth in the urban system of southern Ontario over the period from 1851 to 1971; (2) to determine whether this system has evolved in accordance with the Gibrat process of growth; (3) to contribute to the debate on the relationship between city‐size distributions and economic development; and (4) to offer some technical and definitional suggestions with regard to the rank‐size rule, urban primacy, and the measurement of population concentration. Related Canadian studies covering similar time periods are those of Simmons (1974), who has analysed the growth of larger cities (10,000 and over) at the national scale, and Bannister (1975), who has described the extent of spatial autocorrelation in the growth rates of southern Ontario's incorporated centres. Like Simmons and Bannister we are less concerned with the fortunes of particular places than with the response of the urban system as a whole to expansionary forces. We are centrally concerned with the phenomenon of differential growth (Borchert, 1967; Ward, 1971, pp. 11–49; Muller, 1976, 1977). The fact that towns grow at different rates implies changes in the frequency distribution of city size and in the level of concentration of the urban population. These changes, in turn, have interesting consequences for urban systems t