As of April 1, 1977, Washington's population was estimated at 3,661,975--an increase of 248,725 since 1970. Prepared yearly, this report presents data on the official April 1 population estimates for cities, towns, and counties; components of population change; planned population forecasting activities; procedures which help make the housing unit method of estimation more accurate; comparative mortality levels for counties; county censuses since 1970; and changes in household size (1970-1977). Data were obtained from city and county officials, Armed Forces personnel at major bases in the State, housing directors of the colleges and universities, and the Department of Social and Health Services. Tabular data cover: the classification of cities and counties by population size; annexations by cities and towns; population for highway urban areas, of the Armed Forces, and of selected ethnic categories--Spanish surnamed, Black, American Indian, Asians; components of population change--1920-1977; general fertility rate; abortions by age group and marital status--1971-1976; live births, illegitimate births, and illegitimacy ratio--1965-1976; induced abortions, live births, and ratio of induced births per 1,000 live births in selected states; marriages and divorces--1960-1976; population and housing units in selected counties; average population per household and family--1940-1977; census and postal survey data for selected cities; and population 65 years and over. (NQ)