Serious crime in Fairfax County rose by 6.4 percent in the first three months of this year compared to the same period a year ago, reversing a pattern of decreases in recent years, police reported yesterday.

The Fairfax figure outpaced Arlington, Montgomery and Prince George's counties, which also have reported modest increases in crime in the first quarter of the year.

Arlington said serious crimes rose 1.8 percent, Prince George's County 4.8 percent and Montgomery County 5.6 percent.

Alexandria police reported that serious crime fell about one-half of one percent. The District, reporting on the first two months of 1985, recorded a drop of 12 percent from a year ago. The D.C. figure included a decline of 8 percent in attempted rapes and a 44 percent rise in arsons -- categories not reported in the suburban jurisdictions.

Fairfax authorities said the total number of serious crimes increased from 4,745 to 5,050 in the January-March quarter, led by rising numbers of car thefts and larcenies.

Acting Fairfax County Police Chief John E. Granville attributed the increases to the rapid growth of the county's population and economy, which he said resulted in increased "targets of opportunity."

Granville also said that mild weather during the period may have helped spur a rise in property crimes. "The statistics for one quarter cannot be interpreted as a trend," he said.

Motor vehicle thefts rose to 372 from 259 in the same period of 1984, according to police figures. Larcenies were up by 264, from 3,458 a year ago to 3,722.

Robberies increased by 14, from 124 last year to 138. Granville blamed the rise on a series of robberies of pizza truck delivery personnel in February and March.

"Even though in some instances only pizzas were grabbed and no money was taken, each case is recorded statistically as a robbery, just as if it were a financial institution or a store," Granville said.

Rapes rose from 14 to 17, police reported, but several other key categories continued to decline. Burglaries dropped by 9.9 percent in the first quarter, from 806 in 1984 to 726 this year. Burglaries in the county declined by nearly half between 1980 and 1984, according to police.

There were no homicides reported this year, compared to three a year ago. Aggravated assaults fell from 81 to 75.

In Arlington, there were slight increases in robberies, aggravated assaults, burglaries, larcenies and vehicle thefts. Homicides fell from two last year to one and rapes remained the same at 15.

Rapes and burglaries declined in Prince George's, while vehicle thefts climbed nearly 25 percent and homicides rose from six to eight for a 33 percent increase.

In Alexandria, a 41 percent increase in aggravated assaults and a 22.5 percent rise in robberies paced increases. Homicides declined 50 percent, from four to two. Serious crime dropped overall by 0.46 percent.

Robbery and burglary showed the largest increases in Montgomery County. There were fewer cases of larceny and homicide but more of rape, assault and vehicle theft.