Serious crime in the District declined for the ninth straight month in July, according to police officials who said yesterday that if the trend continues through the remainder of 1985 the District will post the lowest crime level in 19 years.

There were 4,420 Part I crimes -- the police designation for serious offenses such as homicide, rape, arson, burglary and robbery -- reported in July, which is a 4 percent decrease from the same month last year, according to Police Chief Maurice T. Turner Jr.

Turner also released figures showing that serious crime through July was 9 percent lower than for the first seven months of 1984.

"The department is on the verge of accomplishing a milestone," Turner said. "Not since 1966 has the crime rate in the nation's capital decreased so significantly."

Crime in the city peaked in 1969 and 1970, when there were about 83,000 serious offenses reported each year.

The level fell slowly to a low of 49,978 in 1976, then increased again to a high of 68,340 in 1981. Since then, crime in the city has been on a downturn.

Turner said, that based on the 27,890 serious offenses reported in the first seven months of this year, there will be about 48,000 serious crimes for all of 1985.

He said preliminary statistics for August support his conclusion that crime this year will be lower than any year since 1966, when 39,937 serious offenses were reported.

"The reduction reflects the fact that our city's environment has become a wholesome atmosphere for an individual to conduct social or business activities," he said.

Turner's remarks came during an award ceremony at the 6th District police station at 100 42nd St. NE, where Deputy Chief Fred Thomas was awarded a banner and plaque for posting the greatest reduction of crime -- 22 percent -- for the second quarter of 1985.

For 1985 through the month of July, all the city's police district's have had a decrease in crime over the same period last year, led by the 6th District with a 19 percent decrease in serious crime and followed by the 4th District with 15 percent; the 1st District, 13 percent; 5th District, 10 percent; 7th District, 8 percent; 3rd District, 6 percent, and the 2nd District, with a 2 percent decrease.

Crime statistics are viewed by some police officials as an imprecise measurement of the overall level of criminal activity. Drug violations, weapons violations, some sexual offenses, gambling, prostitution, disorderly conduct, traffic violations, numerous white-collar crimes and other offenses that are not Part I crimes, for example, were not included in Turner's figures.

According to monthly reports made by each district, the last time that the city had an increase in crime was October 1984, when serious crimes went up by 1 percent over the same month in 1983.

Turner attributed the reduction in serious crime to "exemplary" police officers and officials, and to such community programs as Neighborhood Watch, Operation Identification, and the Crime Solvers program that has been responsible for the arrest of more than 350 suspects this year.

The success of the Repeat Offenders Project, started in 1982 to target recidivists suspected of committing multiple crimes, and of the D.C. police narcotics task force, which has arrested more than 2,100 persons on narcotics violations in the last 12 months, also was credited with helping to reduce serious crime.