As of April 1, 1976, Washington's population was estimated at 3,571,591--an increase of 158,341 since 1970. This annual report presents data pertaining to: official April 1 population estimates for cities, towns, and counties; components of population change; changes in household size from 1970 to 1976; assessment of accuracy of population estimates for cities and towns; and county censuses since 1970. 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 various divisions of the Department of Social and Health Services. Tabular data cover: classification of cities and counties by population size; annexations by cities and towns; population of selected ethnic categories (Spanish surnamed, American Indian, Black, and Asian American); Armed Forces population; general fertility rate--1930-1976; population 65 years and over; abortions per 1,000 live births in selected states; population change from 1950 to 1970; and household size. Corrected 1970 population data by 5-year age groups for the State and counties is included for the first time. Guidelines for cities to use when considering a special census are also given. (NQ)