Studies of age at time of marriage bear specifically upon rate of population growth and reveal, as well, customs of general social importance. This note presents the results of recent researches in ecclesiastical archives of two localities in Mexico, a country in which reliable population data have been distressingly meagre. Arandas is a municipio (county) in Los Altos of Jalisco, central Mexico, a region inhabited by small peasant proprietors, probably about 85 per cent of whose collective ancestry is Spanish. Its people are strongly anti-agrarista and ardently Catholic; they furnished the bulwark of the powerful Cristero revolution against the Mexican government from 1926 to 1929. The region has been, and continues to be, a source of rapid population increase. According to the census of 1930, its population was 27,624, of whom 7,574 lived in the town of Arandas, the only really important population center in the municipio. San Jose Tateposco, Jalisco, is a small pueblo (village) a few miles distant from Guadalajara. Its inhabitants, who numbered 441 in 1930, are collectively about 85 per cent of Indian stock. Their traditional and contemporary occupations have been the making of pottery, and agriculture; the agricultural activities are limited to cultivation of very small plots, and day labor on the adjoining hacienda. Catholicism is universally accepted, but in recent years the formation of a comunidad agraria by many of its inhabitants who have obtained lands from the hacienda under the agrarian law, has thrown their allegiance to the side of the government rather than to the Cristeros. Railways and good highways penetrate neither Tateposco nor Arandas; both are physically and socially isolated. Marriages of the Tateposquefios have been, and continue to be, almost universally with persons of the same village. The comparatively few marriages in Arandas involving one party from outside the municipio have practically always been contracted with persons from other parts of Los Altos, a region which is homogeneous. In Arandas there are persons of small town and rural middle-class, a type which is absent from Tate-