The line gets really long at Snowball City just about the time that fireflies start flashing in the twilight. That's when customers seeking relief from the season's heat gather in front of the snowcone stand, waiting up to 15 minutes to buy a cupful of syrup-sweetened ice.

Offering more than 70 flavors--from apple to watermelon--Snowball City is an operation worthy of McDonald's or Baskin-Robbins, the only difference being that it sits in the middle of a Howard County subdivision, occupying the elaborately converted garage of a suburban, single-family home.

Chrissy Frentz opened it in 1993 when she was 15. In the seven summers since then, Snowball City has become a community fixture in Scaggsville--no small feat in a county that is so fond of snowballs that local leaders amended their zoning laws to permit stands to dot the landscape.

Mike Budzinski lives next door and can watch the parade of customers from his front porch. "It's like 'Field of Dreams,' " he said. "If you build it they will come. . . . They come from miles around."

His wife, Meredith Resnick, 33, explained the special appeal of Snowball City to the people who live on Cardinal Forest Circle: "There's nothing within walking distance, like in the city. It takes the place of the corner ice cream store or the corner soda shop."

Howard County craves its snowballs. While stands are hard to come by in much of the Washington area, 11 serve up cold refreshment in this suburban county between the District and Baltimore. And those are just the ones that have gone to the trouble to get permits; officials suspect there are probably others operating off their radar screens.

The love of snowballs is in part a reflection of the strong Baltimore aspect to the county's split personality. In Baltimore, snowball stands--operated out of front stoops, basements or garages--are as much a part of summer as steamed crabs and the Orioles.

It's also a business that has been very, very good to the now-21-year-old Frentz and her sister and co-owner Cathy, 19. The money they are raising is helping put them through college.

On a recent Saturday, Chrissy--wearing shorts and an Adidas T-shirt, her brown hair pulled back in a ponytail--prepared the stand for its 3 p.m. opening. As Cathy arranged syrup bottles, Chrissy unloaded four blue Coleman coolers from the family's Dodge Aries, each one filled with 45 pounds of ice she'd picked up at a liquor store.

"We wanted to do it right," Chrissy said of the stand, as she poured scoops of ice cubes into the top of a Koldkiss ice shaving machine. "People are always impressed when they walk up."

It's not hard to see why. Chrissy and her father, Mike, sacrificed the garage to build the stand. At the mouth of the garage--just far enough back that the door still rolls shut--is a blue-Formica counter, topped with the gleaming blue and silver $995 electric ice shaver. On one wall are custom cabinets, fronted with clear plastic sliding doors and perfectly sized for dozens of 3-liter bottles of brightly-colored syrup. A computer, its keyboard covered in protective Saran Wrap, keeps track of receipts.

Snowball City, say the neighbors, is a place to stop at on the way home from the pool, a destination that parents allow their children to bike to alone, the perfect spot for a frozen nightcap during an after-dinner stroll.

"When it's hot, you get that urge for something cold," says Steve Palm, 43, who's driven to Snowball City with daughters Tabitha, 11, and Sabrina, 6. "The biggest problem is deciding what you want."

The selection is overwhelming, and many children stand slackjawed as a parent recites the menu: apple, banana, banana creamsicle. . . egg custard, fireball, fruit punch. . . pina colada, pineapple, raspberry, root beer, root beer float.

After much deliberation, Tabitha and Sabrina decide: blackberry-raspberry and root beer. Dad picks "Skylite," a syrup the color of Windex that is one of Snowball City's best sellers, despite the inability of anyone to describe what it actually tastes like.

Frentz pushes the button on the shaver and it loudly disgorges a Matterhorn of powdery ice into a white styrofoam cup. She gives a long squirt of syrup, melting the ice down a bit, then taps the bottom of the cup on the counter, settling the ice down before adding more ice and more syrup.

Prices start at 35 cents for a tiny bathroom cup-size of ice with a single squirt of syrup and top out at $2.35 for a jumbo, a cup packed with a staggering 32 ounces of ice and larded with eight squirts. It's extra for toppings: sprinkles, M&Ms, Skittles or the strangely addicting marshmallow cream.

Chrissy and Cathy each work three days a week (the stand is closed Sundays). They pay themselves an hourly wage and split the remaining revenue, minus expenses. Both attend the University of Maryland in College Park and have used snowball money to fulfill an agreement they have with their parents: that they pay half their college expenses themselves.

Everything's legal, the result of a 1992 Howard County zoning regulation amendment that specifically allows snowball stands to operate in noncommercial areas. The amendment sprang from an incident seven years ago when some residents of Columbia's Oakland Mills neighborhood complained about the Crazy Ape snowball stand. Moved by a petition from 850 Crazy Ape supporters, County Council member C. Vernon Gray proposed the zoning amendment. Among other rules, the special zoning category stipulates that the stands may sell only snowballs, must be no larger than 200 square feet and can operate only between May and October.

Montgomery County is less forgiving of such homespun industry. "If we're talking about residential zones, it actually is a violation of the law," says Edward Calloway, an investigator in Montgomery's department of permitting services.

One of the few snowball stands in Montgomery County is in Burtonsville, just south of the Howard County line. Mark Sarem, 32, opened it three years ago near his family's farm produce stand on Route 29. He thought he had the proper permits but was told soon after opening that he had 14 days to shut it down. "I stood my ground and basically fought for it," Sarem says. "I told the inspector, 'Look, people are doing it out of their garages.' He said, 'Who and where are they?' "

They were Chrissy and Cathy, so Montgomery County couldn't touch them. Five-hundred dollars and a zoning variance later, Sarem was allowed to keep his stand.

Snowball City will close Aug. 21, before school starts up again. Chrissy recently took a part-time job waitressing in Savage. Cathy has been temping this summer. Eventually they will graduate from college, get real jobs and move away. But the neighbors aren't too worried. Sisters Suzy, 15, and Diana, 12, are being groomed to take over the family business.

There should be snowballs in Scaggsville for years to come.