This study analyzed the role of marriage patterns in accounting for fertility levels and differentials among 2 populations of nomadic Twareg pastoralists (Delta Twareg and Gourma Twareg) and a population of Bambara millet farmers with far higher total fertility. The large differences in fertility between the Twareg and Bambara disappeared when marriage was controlled for, indicating the role of marriage patterns as an intermediate variable. Polygyny and the practice of the levirate (whereby a man inherits his dead brother's wives) lead Bambura men and women to be almost always currently married. The woman is an asset, who is compensated for by bridewealth and is required for herself in her affinal lineage and not only for the children she produces. In Twareg society, where women are not an economic or a labor asset and receive an indirect dowry to support themselves, many men and women never marry and there is a high rate of divorce. Children are a crucial component of marriage for the Bambara; they provide the lineage descendents for the man, support in old age, and labor for labor-intensive agriculture. Noble Twareg women often do not marry until a relatively late age by African standards. Moreover, the considerable periods of time they spend in a nonreproductive state between successive marriages (when intercourse is prohibited) reduces the period during which they can reproduce. Among imrad nobles in Twareg society, household formation is a reluctantly accepted obligation. The availability of slaves removes the economic incentive for marriage. In addition, the powerful kinship solidarity of nobles contrasts strongly with the weakness of marriage relations. Twareg slave women do not have to marry because they are part of a labor and support system that is not dependant on their own household formation. Thus, the complementarity of slave and noble households, slow female entry into marriage, long periods spent outside of marriage, and the powerful kinship system that provides support for women combine to create lower fertility among the Twareg than the Bambara. This analysis points to the role of social institutions such as marraige in fertility.