Mamabu Murakamo, 39, lives in Rockville with his wife Kayako and three children, ages 6 to 11. They are Japanese, here for 18 months while he works as a researcher at the National Institutes of Health in Bethesda.

Yesterday, they were eating Mexican food. Masayo, 6, held a red balloon, and Nahoko, 11, wore a Minnie Mouse T-shirt. "In Japan, we have not so much chance to enjoy getting together with so many country members," their father said.

Their chance yesterday came at Montgomery County's fourth annual ethnic heritage festival, a celebration of the growing diversity of the once largely homogenous county: The foreign-born population nearly doubled in the 1970s. Today, 100,000 residents, or one out of every six in the county, speaks a language other than English at home. The number of Asians and Hispanics in the county's schools has jumped by 50 percent since 1980.

It seemed that almost every office-holding politician was on hand to see and be seen. County Executive Sidney Kramer, referring to his eastern European ancestry, noted that only in the United States "could the son of immigrant parents become the county executive of Montgomery County."

Rain, which threatened throughout the day, held off until late afternoon but nonetheless dampened attendance -- estimated at 10,000 by organizers of the festival, held at Wheaton Regional Park. But the weather did not dampen spirits.

"We're probably the only people who like this weather," said John Griffith, a member of the Welsh-American Society of Washington. "In Wales, we have hot tea."

Throughout the day, children in Greek, Polish and other national costumes danced on two stages. Foods of 17 countries were available for tasting in a large tent, with prices for individual items held to a dollar. Behind a Vietnamese food table, Phan Tung, 32, a computer technician, displayed a banner based on a computer graphics program he wrote. It said, "Welcome to Vietnamese Foods."

There were free sodas, too. "Last year, the line was miles long," said volunteer Nancy Greene, who observed that yesterday's weather was chillier. "They probably would've been better off giving out free coffee."

At the AT&T tent, anyone could call overseas for free. By 3:30 p.m., 125 callers had taken advantage of the promotion. A man called his brother, who was recovering from open-heart surgery in India. A 7-month-old baby "spoke" to his grandparents in Taiwan. A Swedish girl talked to her cousin back home. Other callers phoned Israel, Saudi Arabia, Syria, France, Germany, Norway, Pakistan, Iran, Burma, Canada, Bolivia and Scotland, among other countries.

Up and down the midway, a park path flanked by about 65 booths, could be heard a cacophony of tongues. While Griffith was speaking Welsh to a passer-by, four adults were greeting each other in Polish.

That the Americanization of the new immigrants is proceeding in Montgomery County was apparent at the ESOL (English for Speakers of Other Languages) tent. There, Chansari Yin, from Cambodia, wrote names in Khmer, while her son David, 7, sat nearby with his space laser lunch box.

Not every booth at the festival had an obvious ethnic tie. Raymond Finkleman, who teaches fencing for the Montgomery County and District recreation departments, had a booth promoting the sport.

"What's ethnic about fencing?" Finkleman asked. "It was originated in Europe; how about that? Well, there are some karate people here, too. We're just as ethnic as the karate people are."

As if to prove Finkleman's point, Omar Fernandez, of Fairfax by way of Bolivia, picked up a sword and said to his wife in Spanish, according to her translation, "I'm going to get one of these for when we get into a fight."