Martin Williamson bets he eats at Mario's Pizza House in Arlington about 300 days a year, and he's been going there for 15 years. That's about 4,500 meals at Mario's.

"I miss a day once in a while and occasionally I'm out of town," said Williamson, 41, a landscaper who lives in Merrifield.

On Monday, he stood at the counter inside Mario's, downing a soft drink and chomping on a submarine sandwich.

"Ham, steak and cheese sub," he said. "I always get exactly the same thing. I'm a creature of habit, and this is a happy habit."

The small fast-food operation has stood along Wilson Boulevard virtually unchanged for 30 years. It is owned by Norma Levine, who went into the restaurant business with her then-husband in 1954. The couple operated several Mario's restaurants in the Washington area until deciding in 1958 to devote all their efforts to the Wilson Boulevard location. In 1970, Levine became sole owner of the business.

Today, the carryout is offering the same basic fare it did 30 years ago: pizza and submarine sandwiches. It's still open seven days a week and doesn't close on weekend nights until 4 a.m., one of the few places in the county to remain open that late.

And the routine hasn't changed either. Most customers take their food to go; some choose to eat inside while standing at the counter, or, in pleasant weather, to sit outside at picnic tables.

Glen Rutherford, who heads Arlington's Environmental Health office, the agency in charge of licensing county restaurants, said Mario's is one of the older food establishments in Arlington. He guessed it is one of a handful of the county's estimated 575 restaurants that has developed a large and loyal clientele.

"They're so happy that something hasn't changed," said Levine, referring to her customers.

She estimates the business serves an average of 600 people a day and said most of those are "regulars" -- people who stop by at least once or twice a week.

"Our business is repeat business," she said. "Give the customer quality, then you keep the customer."

Julie Lickiss discovered Mario's soon after moving to Northern Virginia in 1967. She has been a federal government employee in the District for 20 years and estimates she has driven across the Theodore Roosevelt Bridge an average of twice a month during that time for lunch at Mario's.

"The sausage sub -- that's what I always get when I go there," she said. "I like the food and the personal attention. I can walk in there and they know what I want."

"I get the hamburger sub -- there is no better one," said George Farris, another federal government worker in the District who drives to Mario's for lunch several times a month.

"I walk in the door and he's already cooking for me," said Farris, referring to Mario's daytime grill cook, Lefty Lindsay. "Best short-order cook I've ever seen."

Lindsay, 49, has worked at Mario's for 22 years. For much of that time, he's manned the grill, handling the sandwich side of the operation. Typically, he's cooking for a customer before the customer has even stepped out of the car.

According to Lindsay, the operation has a certain fantastic rhythm.

"I've had customers and now I'm waiting on their kids," Lindsay said.

To Joe Williams, the day manager, new customers "stick out like a sore thumb."

"Usually customers have been coming here 10, 15 years," said Williams, who has worked at Mario's 26 years and says he has never missed a day of work.

"Lefty and Joe knew me when I was a kid," said Raymond Leak, 40, a District police officer. "Joe has seen me grow up."

In recent years, other fast-food pizza businesses have sprung up in Arlington, several of which can deliver more quickly than Mario's can. But Levine said her business has not been heavily affected by these. The great majority of her customers continue to stop by, some phoning orders in ahead of time. Levine said Mario's makes about a half-dozen deliveries a day, mostly sandwiches.

Levine says she takes special care to serve a high-quality product. Long ago, she decided to use provolone cheese on pizza instead of mozzarella, which is more commonly used.

"Provolone is a good, rich, smoked cheese," Levine said. "It's tastier."

Levine franchised the business in 1979, eventually opening about 10 operations around the country. But she pulled out of the franchise after several years because she felt out of touch with the product.

"It was hard to oversee and make sure they were running the same quality," she said. "I like my little business the way it is."