Meant as a contribution to the recently initiated debate over household and family composition in the Mediterranean area, the study presents the first results of an investigation of class differences in family behavior in several rural communities in the Segura Basin of southeastern Iberia during a period of agrarian change. A variety of measures and classifications suggest the predominance of the nuclear family, but also that the household formation process and the family life cycle are conditioned by the land tenure system, inheritance practices, and transmission of the patrimony. Also, patterns of mobility and demographic mechanisms exert a powerful influence over family forms and household structures.