An unexpected demographic development in the United States in the 1970's was the shift of nonmetropolitan areas to net inmigration, reversing a 70-year trend. Using the 1970 definition of metropolitan, the percent of the population living in metropolitan areas fell from 69% in 1970 to 67.8% in 1978. No easily identifiable set of reasons explained a majority of moves between metropolitan areas, between nonmetropolitan counties, or between metropolitan and nonmetropolitan destinations. While 53.1% heads of household moving from metropolitan areas to non-adjacent nonmetropolitan counties reported employment-related reasons, such reasons accounted for less than one half of the migration within the entire U.S. Family-related reasons explained between 15.6% and 19.4% of the four types of migration, and various housing and neighborhood considerations explained another 7.1% to 15.8% of moves. The momentum to population growth in the nonmetropolitan sector resulted from the interaction of net migration and natural increase. Net inmigration of persons in their reproductive years may boost rates of natural increase in later periods, causing nonmetropolitan areas to continue to have high rates of population growth. At present the U.S. is still metropolitanizing, but only because of population growth in nonmetropolitan areas and their fusion into existing adjacent metropolitan areas. (NEC)