Unemployment among District of Columbia residents continued to decline in October, reaching a seasonally adjusted rate of 6.4 percent, William R. Ford, director of the D.C. Labor Department, reported yesterday.

The rate was a sharp decline from October 1978, when 7.9 percent of the city's work force lacked jobs. Unemployment in the city for 1979 peaked in June when 9.1 percent of the work force -- including many students out of school for the summer -- lacked employment.

The figures released yesterday reflected the continuation of a seeming paradox in the city's job picture. Although a smaller proportion of city residents lacked work than a year ago, there also were fewer people working -- in actual numbers.

Last year, there were 303,300 actually at work and 25,400 listed as unemployed and actively seeking jobs. This year, there were 299,300 at work but the number defined as unemployed had declined to 20,500.

Those who are unemployed but have stopped looking for jobs are not included in the unemployment total.

Ford said the gap between the unemployment rates for D.C. residents and suburbanites continued to close in October, continuing an 18-month trend. For the entire metropolitan area, the jobless rate was a seasonally adjusted 4.2 percent, compared with 4.5 percent a year earlier.

In September, the D.C. jobless rate was 6.7 percent, compared with 4.4 percent for the entire region.