Dissimilarity indexes were computed for 241 standard metropolitan statistical areas in 1970 measuring the dissimilarity of residential distribution between the population 65 and over and the population under 65. When possible comparable indexes were also computed for the same areas for 1960, 1950, and 1940. The pattern of age segregation in 1970 was analyzed and trends were computed from 1940-1970. The average DI in 1970 was 23.1 with a range from 10.9 to 44.4. The highest segregation was found in rapidly growing SMSAs, particularly in the west and south, and in areas with major military or educational institutions which fostered aggregations of age-homogeneous populations. A trend toward increased age segregation was found, especially in the period of rapid urban sprawl during the 1950s. The main factor conducive to age segregation was growth and differentiation accompanying it, although the presence of higher proportions of aged retarded the process and high proportions of nonwhites accelerated it.