The number of serious crimes reported in Montgomery County rose 6 percent last year, a marginal increase that is largely attributable to population growth in the county, a police department spokesman said.

"We don't feel it's alarming," Sgt. Harry Gehring said of the 1987 crime statistics released by the department last week.

The report showed the number of serious crimes, defined as homicide, rape, robbery, larceny, burglary, aggravated assault and car theft, increased from 25,841 to 27,384 in 1986.

Gehring said the number of reported serious crimes had increased at a similar rate each year since 1983.

There were 18 slayings in Montgomery last year, compared with 10 in 1986, according to the report.

The number of reported rapes rose from 139 to 165, an increase that Gehring said was partly the result of rape victims being more willing to file complaints.

The number of reported robberies, burglaries and larcenies increased from 22,650 to 23,814. The increase, Gehring said, is due mainly to population growth and the growing presence of illegal drugs in Montgomery.

Reported car thefts rose from 2,208 to 2,545.

While the number of reported crimes increased last year, so did the number of arrests, the report said.

For the 25,841 serious crimes in 1986, according to the report, 4,647 people were arrested.

For the 27,384 last year, 5,095 were arrested.