Social capital has often been invoked to explain differences in children's well-being by family structure. That is, developmental outcome for children in lone or step parent family is not at par with that of children from intact family because parental investments on children may be lower not only in financial and human capital but also in social capital. This proposition has been difficult to examine in greater depth because of lack of conceptual clarity and of data to measure social capital. Using a definition of social capital as the "ability to secure benefits through membership in networks and other social structures," we focus on the impact of family structures on social capital engendered by three types of networks: (a) "informal" ties with kin, families, friends, neighbours, and workmates; (b) "generalized relationships" with local people, people in civic groups, and people in general; and, (c) "relationships through institutions." In particular, we examine differences in the measures of social capital among women living with no children in various marital arrangements, and women living with children in intact, step, and lone parent families. Data from the Canadian 2003 General Social Survey on Social Engagement confirm that social capital is indeed greater in intact families than in lone parent families. Mothers in intact families (especially married mothers) have larger informal networks, are members of more primordial and purposive organizations, have greater trust in people in the family, in the neighbourhood, and in people in general, and have greater confidence in government or business institutions. In general, social capital of mothers in step families is in between that of married mothers in intact families and lone mothers. Thus, the assumption in the literature that family structure can serve as a proxy for social capital may be justified. However, this study contributes a unique way of measuring social capital in terms of networks if and when data are available and a way of investigating the relationship between family structure and social capital; that is, the former as a determinant of the latter.