The U.S. Census Bureau released yesterday its official 1980 population count for the District of Columbia -- 635,185, a drop of more than 121,000 or 16.1 percent since 1970, but about 67,000 more than the incomplete preliminary figures given to city officials in early August.

Despite the sharp population loss, the Census Bureau said Washington has 578 more housing units now than it did a decade ago, indicating a major decrease in average household size here as single adults and childless couples replaced families with children.

When city officials received the earlier census numbers, which help determine federal aid allocations, they said the population figure for the District was far too low, and Mayor Marion Barry said the city would "consider an official request for a recount."

Late yesterday Alan Grip, press spokesman for the mayor, said Barry would not comment on the new official count because the city had not yet received it from the Census Bureau.

Elsewhere in the metropolitan area, the Census Bureau reported that the 1980 population of Montgomery County was 574,106, an increase of 9.8 percent since 1970, but far slower than the 53 percent spurt during the 1960s.

The 1980 population of Gaithersburg was 169 percent higher than it had been 10 years earlier, but in Rockville, the bureau said, the population rose only 2.1 percent during the decade.

No complete figures were available yet for any other parts of Montgomery or for Prince George's county. In the Virginia suburbs, the only figures released were for Fairfax City, whose population dropped 14.6 percent during the decade to 19,404 this year.

On Sept. 25, U.S. District Court Judge Horace W. Gilmore ruled in Detroit that the traditional census head count could not be certified for the reapportioning of Congress by Jan. 1, as scheduled, unless it is adjusted in all parts of the country to compensate for what the judge found to be a widespread undercount of blacks and Hispanics.

The Census Bureau has asked that the ruling be appealed on grounds that there is no scientifically valid way to adjust the count for individual cities and counties.

Adjusting the census head count figures has been pushed strongly by a phalanx of big city members of Congress and mayors, including Barry. They are fearful not only of losing seats in Congress and the state legislatures but also of getting less federal aid, much of which is distributed according to formulas that include population.

Even though the District of Columbia has no vote in Congress now, the District's population is a sensitive issue in the fight for a constitutional amendment that would give D.C. full congressional representation. With 756,668 people in the 1970 census, the District of Columbia was larger than 10 of the 50 states. If the new figure of 635,185 is certified without major adjustment, it seems likely that only four states will have fewer people than D.C. in the 1980 census.

The census count gives the population as of April 1.

Census director Vincent P. Barabba said the figures released yesterday contain all the people who sent back questionnaires mailed out at that time plus others found by census takers who visited individual homes in spring and summer before census field offices closed last month.

The numbers are still considered preliminary, Barabba said, because minor errors in arithmetic may yet be found. But, he said, "This is basically the count." He said the release did not violate Judge Gilmore's order.

Barabba said he had "apologized to (Mayor Barry's) office" when he learned that the Census Bureau had released the figures to the press instead of following "our normal procedure" of informing the local government before information is made public. But he said the numbers released yesterday are correct.

The earlier figures, which put the city's population at 568,300, were incomplete "interim working numbers," which were given to D.C. officials in early August so they "could be involved in finding places and people we may have missed," Barabba said.

"D.C. did an outstanding job of this," Barabba said. As a result, the Census Bureau found 278,904 housing units, about 20,000 more than had been counted by midsummer and slightly above the city government's own estimates. The new population count is about 15,000 shy of the 650,000 that city officials had anticipated -- a difference of 2 percent. The incomplete preliminary figures had been 82,000 below the city government estimates.

The new population count for Montgomery County is just 10,000 more than the Census Bureau's preliminary figure. Officials said the increase was far less than in the city because the number of questionnaires returned had been much greater in the county and because Montgomery has fewer low-income apartments.