More than twice as many businesses opened in Virginia's Loudoun County last year as in 1984, the vast majority of them in the county's booming eastern area near Dulles International Airport, according to a new report by the county's Department of Economic Development.

Some officials this week trumpeted the arrival of 67 new county businesses as further evidence that Loudoun, which has been ambivalent about development because of its effect on the county's rural life style, is coming into its own as an economic heavyweight. Loudoun Supervisor Frank Raflo, however, cautioned that with growth comes new burdens, particularly on already clogged highways such as Rtes. 28 and 7.

Unlike the year before, last year's growth primarily came in small- and mid-sized companies that will be providing services to residents and larger companies. Economic Development Director June Bachtell Wilmot hailed this sign as evidence that Loudoun is developing a more mature, self-supporting economy, one in which residents and large businesses can have their demands for goods and services met without leaving the county.

"When you have a Magnavox or a Xerox," both large commercial projects under way in Loudoun, Wilmot said, "how do you provide support services to them? We're making progress there."

But the most encouraging aspect of last year's growth, county officials said, was that business expansion was able to outpace Loudoun's boom in residential development, giving strength to the tax base.

The balance between business and residential development is considered crucial for local governments because houses usually generate a demand for more county services than they provide in tax revenues, while the opposite generally is true of businesses.

This balance is especially important in Loudoun, where the county counts on high property-tax revenues from businesses to offset losses that come from tax preferences given to farmers, officials said. Agriculture remains Loudoun's largest industry.

About 18.5 percent of Loudoun's tax base last year came from property taxes on businesses, up about 2 percent from the year before but significantly short of the county's goal of 25 percent. Fairfax County collects about 20 percent of its tax revenue from businesses, while Prince William County lags well behind at about 8 percent.

Wilmot said it was noteworthy that much of last year's growth was fueled by firms moving from more congested Washington areas. "When one area gets filled up, it's natural to look to the next stop west," Wilmot said.

Loudoun Supervisor Andrew R. Bird III lauded the growth figures and said they are a partial reflection of what he believes is a fundamental shift in the county government's attitude toward business. "I'm pleased whenever we have that size of increase in the county. My only misgiving is that it is still too small," he said.