The number of serious crimes in the Washington area's two wealthiest counties rose -- dramatically, in some categories -- during the first half of this year, officials in Fairfax and Montgomery counties said yesterday.

Police and prosecutors attributed the jump in several of the seven crime categories that law enforcement officials classify as "serious crime" to increased population and heightened commercial activity in both of the rapidly growing jurisdictions.

They also emphasized that numbers in some categories, including murder and rape, remained stable or fell during the first six months of 1985, compared to the same period in 1984.

In Fairfax County, the number of serious crimes reported to police rose by 1,181, or 11.9 percent, to a total of 11,041 crimes.

The increases occurred in robberies, larcenies and motor vehicle thefts, police said. There were decreases in the numbers of murders, rapes, aggravated assaults and burglaries.

The increase was spread more evenly in Montgomery County, where police reported a rise in all crime categories except rape.

The county had 50 reported rapes in the first six months of 1985, compared with 69 in the first half of last year. The total number of serious crimes in the county rose by 540, or 4.9 percent, police said.

Increases in Fairfax were led by auto thefts, up 19.6 percent, and larcenies, which rose 15.1 percent. In Montgomery County, auto thefts increased by 20.1 percent and robberies by 19.4 percent.

Montgomery Police Chief Bernard D. Crooke said he was concerned about the increase in robberies, especially in the category of armed robbery, and said police "will continue to address the problem with special assignment team officers working closely with the investigators to apprehend the perpetrators."

The statistics do not compare the severity of different offenses, said Fairfax County police spokesman Warren R. Carmichael. "One larceny, one burglary, one rape, one murder -- they all count the same."

Carmichael noted that growth in commercial and residential development pushes certain types of crime, especially thefts from construction sites, upward. "They're taking everything from boards to bathtubs and dryers to drywalls," Carmichael said.

Montgomery County police spokesman Sgt. Harry Geehreng also blamed the rise in crime on development growth and predicted that county crime statistics will continue to rise in the future.

In Fairfax County, where reported crime fell between 1982 and 1984, chief county prosecutor Robert F. Horan Jr. said that trend may have halted. "I knew the [decline in crime] had to bottom out -- particularly with property crime," Horan said.

Fairfax County's population rose by about 19,000 in the past year to a current total of 668,290, according to county records.

Montgomery County's population grew by 13,000 to 623,000 from April 1984 to last January, according to the latest available figures.