This economic analysis of migration was designed to test the following six hypotheses on the causes of out-migration from agriculture and its effect on the rural economy of Western Nigeria: (1) age of the rural family, of which the migrant is a member, is positively related to rural-urban migration rate; (2) a positive relationship exists between rural-urban migration rate and migrant education level; (3) distance between source region and receiving urban centers is negatively related to migration rate; (4) rural-urban migration is a positive function of the urban-rural earnings gap, weighted by the probability of securing urban employment; (5) availability of urban relatives is positively related to rural-urban migration rate; (6) rural-urban migration is a response to the"attractiveness" of urban areas. Data were collected from 180 families in six villages and 480 migrants from these 180 families who were living in urban Western Nigeria during 1971-72. Policy implications suggested the need to narrow the urban-rural earnings gap; invest in education tailored to labor demands of urban areas; and develop agricultural credit. The effect of migration on the rural economy was associated with increased outlay of hired labor, increased farm size, higher rural earnings per head, and a net transfer of capital. (JC)