A number of recent surveys show that fertility has begun to decline in Botswana, Zimbabwe, Kenya, and southern Nigeria. This study of an urban area in southwest Nigeria confirms a fertility decline and throws on the erosion of traditional supports for high fertility. The authors conclude that the sub-Saharan fertility transition is likely to differ during its early decades from the pattern established by the European and Asian transitions: the greatest demand for fertility control will come not from older women wishing to cease family building but from young married women who wish to maintain or lengthen traditional birth intervals even though the traditional mechanisms for achieving those ends are decaying. The onset of fertility decline is likely to be determined by the attainment of relatively low levels of infant and child mortality, substantial extension in female secondary education, an ample supply of contraceptives, and government leadership toward controlling family size.