Nineteen of the nation's 25 largest cities - including Washington - lost population between 1970 and 1975, the U.S. Census Bureau reported yesterday.

The loss in Washington amounted to 45,150 residents - or 6 per cent - and brought the city's population down to 711,500. The District of Columbia now is the 12th largest city in the U.S., the bureau said, down from ninth place in 1970.

The big city population losses occurred in all parts of the country, including Los Angeles and Dallas, which until 1970 had experienced rapid growth rates.

However, the south west and west were the only areas with big cities that continued to grow after 1970, and they now account for half the nation's 10 biggest cities - Los Angeles, Houston, Dallas, San Diego and San Antonio.

Washington's 6 per cent loss was the same as that in Baltimore, which is still the nation's seventh largest city with a population of 851,700.

Detroit, still the country's fifth largest city, lost 11.8 per cent of its population from 1970 to 1975, while Cleveland, which dropped from 10th to 18th among the nation's big cities had a 15 per cent loss.

Frederick Cavanaugh, chief of the Census Bureau's local population branch, said the widespread population losses probably reflect the continued movement of city residents to the suburbs.

But he added that many metropolitan areas, particularly in the Northeast and Midwest, have lost population since 1970, as rural areas and small towns have grown substantially for the first time in decades.

Throughout the United States, population grew by only 4.5 per cent during the first half of the 1970s as the birth-rate fell to the lowest point ever. Virtually all the growth was in the South and West.

Cavanaugh noted that some big cities, including New York, already had a sizeable net out-migration during the 1960s. He said their population grew slightly because of the substantial surplus of births over deaths. With births down, however, Cavanaugh said, there cities are now showing population losses.

In at least one case, he said, the borough of Manhattan, in New York City, deaths exceeded births by 7,500 between 1970 and 1975.

Cavanaugh said the widespread population loss in big cities first showed up in estimates for 1973 that the Census Bureau prepared for the first federal revenue-sharing legislation. He said the new estimates indicate that the pattern is continuing, and probably will lead to reduced allocations for many big cities when revenue sharing funds are distributed next year.

Among the cities with particularly large five-year losses are St. Louise, down 15.6 per cent to 525,000; Pittsburgh, down 11.8 per cent to 458,700 and Cincinnati, down 9.1 per cent to 412,600.

The population of New York, which is still the nation's largest city, fell by a relatively modest 5.2 per cent, although the loss totaled 413,950 in five years. According to the new estimates, New York had 7,481,600 people in 1975.

The new census report gives estimates for all 162 American cities with a population of at least 100,000.

Among the bigh gainers were Anchorage, Alaska, up 2 per cent to 161,000; Las Vegas, up 2 per cent to 146,000 and Aurora, Colo., a suburb of Denver, up 54 per cent to 118,000.