ACCORDING to the preliminary count of the Census Bureau, population in the District of Columbia in 1990 has fallen below 600,000 to 574,844. This level represents a 60-year low for the city, but District officials believe there is reason to question the exactness of the Census Bureau data and are gearing up to challenge the count. On this score, they may have a point. Only two years ago, the Census and the Greater Washington Research Center were estimating the city's population at 617,000. Admittedly, the last two years haven't been exactly salad days in the nation's capital, but it's hard to believe that more than 42,000 residents have migrated from the city in that short span of time. Also, District government and Census Bureau estimates have clashed before, and when it counted -- in 1980 -- the District's challenge was successful and the final tally was increased.

Since census figures help determine how much federal aid is received by localities, the Census Bureau recognizes that much rides on the final count and, therefore, gives these preliminary counts to be reviewed and challenged. So the city will get its day in court and stands a good chance of winning. But something else is at stake here besides federal aid. Though the rate of the District's population decline may be disputed, the fact remains that while surrounding jurisdictions are experiencing population growth -- a sign of vibrancy -- the city itself forecasts a continued loss of people. And this should be a basis of official concern as well as worry for those residents who remain.

From a high of about 800,000 in 1950, the District' population has been on a downward slide -- a sharp loss of more than 118,000 between 1970 and 1980 -- and a steady erosion especially during the last five years. And those who are giving up the ship are some of the very citizens the District needs most -- those mature men and women, many with families, who are either near or close to the top of their best earning years. This is a subject that the next D.C. government is going to have to address and a situation that it should be committed to reverse.