In the early 1950’s, more deaths than births began to occur in a few counties of the United States. The phenomenon has since spread, especially in the 1960’s. In 1966 it occurred in 271 counties, and had affected a total of 324 counties since 1950. Natural decrease had earlier been present in some areas of the United States during the 1930’s. At that time, it was primarily associated with population of low intrinsic fertility. Its occurrence since 1950, however, has not been related to inadequate childbearing rates, but rather to the development of distorted age structures. In most eases, the distortion is the product of prolonged and heavy outmigration of young adults to the extent that the number of couples of childbearing age remaining in an area is insufficient to produce births in excess of the number of deaths occurring to the larger older population. In a minority of cases, the excess of deaths has resulted from large in-migration of older people into retirement areas. The occurrence of natural decrease has been most heavily concentrated in the central part of the country, especially in marginal Corn Belt areas of Missouri, Kansas, Nebraska, and Iowa. It has also been common in central Texas. The counties affected are typically rural and agricultural, with a high median age, a history of population decline, and average population fertility. They had a total 1960 population of 4,003,000 persons. It is projected that an excess of deaths over births will have affected 520 or more counties by 1970, or about a sixth of all counties in the Nation. Given the increasing extent of the condition, research appraising its social and economic consequences for an area is urged.