Standing at the front door of Capt. Billy's, greeting the lunchtime arrivals at the locally famous crab house, was The Man himself.

Billy Robertson, 69, had already raced up to Baltimore and back, 140 miles round trip, for a dose of radiation at Johns Hopkins University Hospital, a daily ritual since his cancer was found earlier this year.

But Robertson, dressed in khaki shorts, knit shirt and a gold-and-diamond necklace bearing the Capt. Billy's crab logo, was back yesterday in plenty of time to josh with the hungry when his crab house, an institution in Southern Maryland, opened for business at 11 a.m.

Although Mondays are the slowest day of the week, a steady flow of customers poured into the crab house, a single-story, vinyl-covered building that sits on a wooden pier that juts into into the lazy, broad waters of the Potomac.

That's the way it goes with a crab house in the summer, and Capt. Billy loves it. "There's no letup. It's a seven-day week. It's solid gold--you don't quit. Come September . . . we'll get a break."

Outside it was baking and there was the stink of seafood, and the big Route 301 bridge two miles downriver shimmered in the haze like a hallucination. But heat's good for business, too. "The wives don't want to stay home and cook," said Robertson in his distinctive drawl.

He's been in the business since age 9, when he began crabbing off Popes Creek in Charles County, where the restaurant now sits. Everyone who comes to Capt. Billy's knows the story because it's on a sign at the front door. With the $21 he earned that summer, Billy bought a rowboat "and so embarked on a seafood career," the sign informs.

As Robertson manned that door yesterday, customer Elsie Mae Harrison, of La Plata, gave him a hug and marveled at the short white growth that has sprung up with vigor on his scalp, replacing the hair lost to chemotherapy. "It's like I'm in the Army," cackled Capt. Billy.

At the register, Pat Goldsmith, a burly, bearded waterman with a crab tattoo, groused about the $50 he got for a bushel of crabs he'd just delivered. Quick as a flash, Capt. Billy gave him an extra $10.

Goldsmith wanted to know if Capt. Billy will be paying $60 a bushel next week, too.

Capt. Billy demurred. "I can't promise you that," he said, grinning. "It all depends on what the crabs do. Quality, quantity--that's the way it's going to be."

Prices are running $20 a bushel higher than normal because of a shortage of crabs.

"When we have to pay those prices, we have to charge accordingly," Robertson said.

But there's been no letup in business. In the afternoon, he was on the phone with his suppliers, making sure he'll have enough crabs for the weekend, when thousands of customers will come through his doors, arriving by land and water.

"It seems to me they'd get tired of looking at me," he said, "and tired of eating crabs. But it hasn't happened."

CAPTION: After chemotherapy, Billy Robertson discusses a secret recipe.

CAPTION: Business at Billy Robertson's restaurant hasn't let up.