The objectives of the study are : (1) to ascertain the concomitants of nuclear and extended families in industrializing societies, and (2) to determine cross-societal regularities regarding these factors. The analysis was based on data collected in sample surveys of three metro pol itan areas : Buenos Aires, Rio de Janeiro, and Santiago. The variables considered in the investigation are: age, social status, foreign extraction, migration, and social mobility. Multiple discriminant analysis is used to determine the effects of the five variables, when taken simultaneously. They account for more than 50 per cent of correct classification of nuclear and extended families in three cities. It is concluded that the model used predicts family composition in these urbanizing societies, satisfactorily. However, a common configuration of factors relatad to three-generation households is not found across societies. Although no common pattern is ascertained, each variable is significant in one city or the other. The importance of each specific element may be related to the stage in which these societies are in the formation of their urban-industrial structures. In the process of societal change migration is significant for the nuclearity of the family in two cities and is third most important in the other urban center studied. The relevance of this phenomenon confirms expectations since migration is an intrinsic part of urbanization.