This paper is concerned with the question of how the social and economic structure of cities affects the degree of political competition and how in turn these factors affect the degree of a political stability. It isbased on a comparative empirical study of the outcomes of the communal elections of 1952, 1958 and 1964 in 147 Belgian cities that had a population size of 10.000 or more in 1947.In the first place the following generalizations are made with regard to the election outcomes in these cities during the 1952-1964 period. 1. A general proliferation of lists or parties participating in the elections from 1952 to 1964. 2. A trend from one-party-control over the college (executive committee) of the city council toward coalition-control. 3. A net increase in the number of catholic lists and a net decrease in the number of socialist and liberal lists participating in the colleges of these townships. 4. An increase in the number of cities in which newer, smaller and nontraditional parties or lists participated in the college of the city council.Three measures of political competitions are employed: 1. The average number of parties or lists, that entered the communal election of 1952, 1958 and 1964. 2. The average number of lists that received at least ten per cent of the vote in these three elections, and 3. The presence or absence of a coalition on the college of the city council in 1952.The two measures of political stability, which are also based on the results of the same elections, are: 1. The degree of stability in the lists and parties participating in the college of the city council, and 2. The degree of stability in the list or party controlling the college of the city council. In general, measures of structural differentiation, linguistic diversity, industrial diversity, and social heterogeneity (i.e. , the presence of a large middle class) are found to be positively related to the degree of competition in local politics. In turn, measures of each of these structural factors and the measures of political competition are found to be negatively related to measures of political stability. Regression analysis supports the interpretation that diversity and heterogeneity in the social structure of cities - specifically, population size, density, and the presence of many persons with high occupational status - contribute to greater political competition in local polities, but that it is the degree of political competition that most strongly affects the degree of political stability.The conclusion is drawn that cities with a high degree of social and economic heterogeneity have a greater amount of conflict and cleavage. This results in greater competition in the political arena whichin turn predisposes such cities to have a high degree of instability in the control of their city governments.