The upward turn in Washington's white population appears to be slowing, the District government has reported, while the city's black population continues to decrease.

According to new city population estimates, Washington had 173,800 whites in mid-1978, about 1,500 more than a year earlier.

Over the previous two years, however, the numbers of whites had climbed by 9,600 after a steep decline that lasted more than a quarter of a century.

Meanwhile, Washington's black population was 502,300 in 1978, down 18,900 from 1977 and down 62,900 from its peak in 1972.

Overall, blacks made up 74.3 percent of the city's 676,100 residents in 1978, according to the report. This percentage was down from a high of 77 percent black population in 1975, but according to the U.S. Census Bureau, still was greater than the proportion of blacks in any other major American city.

Gangu Ahuju, the D.C. government's chief demographer who prepared the report, cautioned that year-to-year fluctuations in estimates are too uncertain to indicate if the apparent back-to-the-city movement by whites is actually ending.

But Ahuja noted that although more whites are buying rowhouses near downtown and Capitol Hill, the white increases in these neighborhoods are partly offset by a slight decline in the number of white children and elderly west of Rock Creek Park.

Among blacks, the steepset decreases continue to be among children under 15, reflecting both a sharp drop in births and an increase in the movement of black families to the suburbs.

Since 1972, the number of black children under 15 has declined by about 22 percent, while blacks aged 15 to 64 have decreased by just 5.8 percent. The number of D.C. blacks over 65 grew by 10 percent in the same six years.

Among whites, Ahuja pointed out, the 30 to 40-year-old group increased substantially last year, while the number of whites in their 20s, which had grown in previous years, stayed about the same.

"Houses are getting so expensive now," Ahuju said, "that it's mostly the people over 30 who can afford to buy them." Also, he noted, many whites over 30, who several years earlier have married, had children and moved to the suburbs, now are remaining single or married and childless and are buying houses in the city.

The proportion of females among D.C. residents of both races continues to increase, according to the new report.

Overall, 55.2 percent of all city residents were female in 1978, compared to 53.5 percent in 1970.

Under age 15, there are slightly more boys than girls, but for every group older than that there is a decisive female majority. Among D.C. residents in their 50s, women make up 56 percent of that group, and among those over age 65, women account 64.2 percent

In 1970, the last year when an official census count was taken, Washington had 545,632 blacks and 210,878 whites for a black proportion of 72.1 percent of the total city population. In the 1960 census, blacks accounted for 54.8 percent of D.C. residents.

Ahuja said the new population estimates are based on birth, death and school enrollment records. He said the recent decline in births and the trend of single persons and small families moving into the city has "tended to weaken to some extent" the reliability of the estimates by race, particularly in the 20 to 34 age group.