101 N. Market St., Frederick, Md. 301/663-1266. Open seven days a week for lunch, 11:30 a.m. to 2:30 p.m., and dinner, 5:30 p.m. to midnight. AE, V, MC, DC, Choice. No separate smoking section. Reservations suggested. Prices at lunch, $2.95 to $6.95, at dinner appetizers $3.95 to $6.95, entrees $11.95 to $21.95. Full dinner with wine, tax and tip about $80 for two.

FREDERICK STILL MAKES MUCH of its historic invasion, but now there is a more welcome invasion: of restaurants. We counted five new ones within a block on Market Street, and our random sample was a winner. Louisiana Purchase makes us surer than ever that the Cajun craze may last. The crawfish bisque was thicker than it should be, and tasted unexpectedly of vinegar, but it was all the better for the tang, and it had the hot-sauce heat and the complex spicing that makes Cajun food addictive. Barbecued shrimp, too, were unorthodox -- with the peppered butter sauce on the side -- but plump and fresh and fine, the jumbo shrimp in the shell bedded on rice and incendiary once you dipped them in the butter. We also applauded a Cajun Style Seafood Platter, which topped good batter-saute'ed redfish with crab and a mellow sauce meunie re, nestled with four shrimp and four oysters, both cooked just right. Cajun Prime Rib -- spiced and blackened -- was a wonderful dish that could eclipse the blackened fish fad. The house salad dressing is a brightly flavored Creole mustard mixture and the crisp mixed salad is generously topped with crab meat. The red beans and rice dish uses kidney rather than Louisiana red beans but isn't not bad, and baked potato could have replaced the red beans. The menu also lists gumbos (unfortunately, the kitchen was too rigid to serve appetizer bowls of gumbo, so I can't report on them), jambalayas, etouffe'es, other blackened dishes, a few non-New Orleans dishes and daily specials. The wine list is minimal but has some spicy whites to match the food. On the dessert list are pecan and blueberry pies, cheesecake and Mississippi Mud Pie, plus New Orleans Pecan Ice Cream Ball, an astounding dessert of fried ice cream, molasses, whipped cream and pecans.

The agreeable serving staff makes up for the kitchen's surliness, and the dining room has an attractive New Orleans simplicity, with white cloths and candlelight. There is also a grand piano, which was put to use the evening we were there. All this plus prices like yesteryear's compared with Washington's: $15 for blackened prime rib, $12 for chicken and up to $17 for the seafood platter. ENGLISH'S

ORDINARILY I WOULD avoid a chain restaurant if I could find a homestyle eating place on the road. But in Salisbury I had a yen for fried chicken, and I got a universal recommendation: English's, a local chain, 53 years old, with 16 branches around Maryland. It sells about two million pounds of chicken a year, all bought fresh daily and breaded, then marinated, in its central commissary.

*Three of the English's chain are waitress-service restaurants, the two in Salisbury in old diners, the other in Westover; two others (one in Salisbury and one in Ocean City) are cafeterias. The rest are self-service restaurants.

Dinner at one of English's waitress-service restaurants? Good food, good fun and great satisfaction. Dinner at a self-service branch? Marginal food, a dismal dinner that left a bad taste. The difference? The human touch. At English's, you are where you eat as well as what you eat.

At all the restaurants the fried chicken is the same. Its meat tastes fresh, picks up the flavor of the marinade, and has the juicy, firm texture of chicken properly fried rather than steam-fried in a closed fryer as is much fast-food chicken. The Day-Glo orange tinge to the crust was a bit disconcerting, the texture hard rather than simply crunchy. But it was good. When I told the waitress in Salisbury that one piece was oversalted, she insisted on replacing it.

At all the restaurants the crab cakes are identical. They are not classic crab cakes with lump crab meat lightly bound, but are more like crab croquettes, the shredded meat whipped with filler into a light fluff, then deep-fried. If you are willing to accept something close to crab-flavored turkey stuffing as a crab cake, they are nicely seasoned and taste appealing, have a light creamy texture and a crunchy surface and can hardly be faulted at $2.35 for a sandwich.

Then the differences take over. At the wonderful-looking '30s diner where I first dined English's style, I could choose from a long menu that included fried fresh oysters or flounder in season, and best of all a platter that included two pieces of chicken, a crab cake and a slab of ham plus two vegetables and a big, puffy, delicious sweet potato biscuit (which improved the indifferent ham when it and the two were sandwiched as a ham biscuit). The waitress mothered us, urging dessert on us and serving the best iced coffee in recent memory. We left, eager to bring the family to English's (children under 12 eat free, from fried chicken through chocolate chip cookies).

So I tried English's again, this time a self-service branch in Cambridge. Yes, the crab cake and chicken were identical. But here the fried seafood is all factory-made and frozen, there is no iced coffee (or even lemonade the day I asked), and the drinks come in paper cups. The fast-food look of orange plastic and wood-grain Formica with hanging plants and salad bar is less charming than the lovingly polished art-deco chrome of the diner, but the differences are more. The sweet potato biscuit was puny and undercooked. The counter help didn't know the difference between apple dumplings and apple cobbler and insisted they were the same. The garnishes were gone; the concern about whether you enjoyed your meal was gone. And the on-premises cooking included such travesties as a gloppy plateful of chicken and dumplings with dried-out chicken, dumplings soggy from grease and a faintly orange gravy that quivered on the fork and tasted oddly like peanut butter. Home cooking gone awry. Crab chowder lacked all but the faintest crab, but even as a tomato-vegetable soup it was unpleasant.

The head office of English's tells me that each branch does its own cooking except for the chicken, crab cakes, cobbler and dumplings. The crab chowder and sweet potato biscuits also come from the central commissary, but clearly they are finished off at each location.

Thus, for the same $6 to $7 dinner you can find fresh flounder, soft-shell crabs and fried oysters in season, with care in the cooking and the service, in a classic diner. Or you can get distracted, unconcerned counter service with food badly cooked and glopped on a plate at a self-service fast-food eatery. English's teaches us the difference between what is possible and what is probable.