Serious crime in Arlington dropped 17 percent in 1982 over the previous year, falling to its lowest point in nine years, county police said yesterday.

The falling crime rate is part of a nationwide trend reflected in other suburban jurisdictions here. Fairfax County, experiencing its lowest crime rate in 10 years, reported 11.6 percent fewer crimes last year than in 1981. Fairfax City experienced a 27 percent drop over the same period and preliminary reports in Alexandria indicate a 6.2 percent decrease.

While rapes and aggravated assaults in Arlington rose 11 percent and 6 percent, respectively, all other categories of serious crimes reported annually to the FBI decreased, most notably burglaries, which plummeted 33 percent, police spokesman Tom Bell said.

Overall, there were 7,905 serious crimes in Arlington in 1982, compared with 9,537 in 1981 and 7,204 in 1973, Bell said. Police resolved nearly 28 percent of last year's crimes.

"Naturally I'm very thankful that it's not a 17 percent increase in overall serious crime , but a 17 percent decrease. But by no stretch of the imagination am I satisfied," Police Chief William K. Stover said yesterday. "We haven't won the war yet."

Stover attributed the decrease to several factors, including the increasing willingness of citizens to testify and create such crime-fighting, anti-burglary groups as Neighborhood Watch.

According to police figures, homicides remained the same, at six cases last year and in 1981. There were 61 reported rapes in 1982 and 55 the previous year. Robberies were down 16 percent at 320 cases; assault, up 6 percent at 267; burglary, down 33 percent at 1,445; larceny, down 13 percent at 5,318, and auto thefts, down 18 percent at 488.