The past 30 years have witnessed rapid and significant advances in the Mexican economy. Yet, even though mortality has fallen precipitously, recent investigators have looked in vain for signs of a transition in Mexican fertility. For the country as a whole, no decline is evident.1 Moreover, available evidence indicates that urban fertility has tended to increase in recent years even though it remains lower than rural fertility.2 Rather than signify that no transition in fertility has occurred, however, these results may indicate that the effects of modernization have not been sufficiently pervasive to be reflected at a nationwide or urban level. Since existing theory maintains that fertility declines begin in the upper educational and occupational strata of large urban areas, an adequate assessment of the possibility of a change in Mexican fertility requires the investigation of fertility differentials among socioeconomic subgroups within urban areas. This paper is a first report of such an investigation. The data are based upon a survey of 1,640 males between the ages of 21 and 60, resident in the metropolitan area of Monterrey, Mexico.3 The Monterrey metropolitan area serves as an excellent location in which to investigate differential fertility because of its size (an estimated 950,000 inhabitants in 1965) and its importance as a center of Mexican heavy industry (43 per cent of the economically