Open for lunch Tuesday through Friday 11:30 a.m. to 2:30 p.m., for dinner Tuesday through Saturday 5:30 p.m. to 1 a.m., Sunday brunch 11 a.m. to 3 p.m.. AE, CB, DC, MC, V. Reservations suggested on weekends. Valet parking for dinner. Prices: At lunch, main dishes $3.25 to $8.25; at dinner appetizers $3.15 to $6.75, main dishes $9 to $17, desserts $1.50 to $3.25; Sunday brunch $10. Full dinner with wine, tax and tip about $25 to $30 a person.

Yes, Maison Blanche, there is another Washington. Joplin's, on the second floor of the Howard Inn, may not be downtown, but it certainly is a downtown restaurant. In fact, it could be downtown Manhattan, for it is closer than perhaps anything in Washington to being a supper club.

Start with the pianist, Frank Hinton, who sings mellow and personalized music--a little Scott Joplin but mostly slow and easy popular classics. His trio plays and is alone worth a detour, as Michelin might say.

Then consider the dining room, with tables raised around the dance floor, set with burgundy cloths and bordered by highly polished brass rails. Along one wall are wonderful fanciful Joplin posters; another wall is banked with plants, enough for a rain forest--not just a few to give a California grassy feeling. The entrance is flanked by etched and frosted glass panels, the dining room is dim and romantic, furnished with curved booths and trimmed with flowered brocades and burgundy fabric wall coverings. Waiters in black tie serve with such smoothness and flair that it seems theater and very well directed.

The menu is a mixture of Europe, New York and the South. You can find gumbo and escargots, flaming beef brochette and fried catfish, coq au vin and barbecued ribs. The key to success, though, is that the chef has an instinct for cooking seafood. And when he turns seafoods into soup he is indeed outstanding. The crab soup is mysteriously creamy without being cream-rich; it has a delicious and unique flavor. Gumbo, called a specialty of the house, is only slightly less outstanding, needing a dash of hot sauce to liven the shrimp- and rice-filled bowl of green stew-soup. There is also a red snapper chowder that floats exceptionally good chunks of fish in a strong tomato broth.

Other appetizers are straightforward shrimp, crab, clam or oyster cocktails plus clams casino and oysters Rockefeller, and they reiterate the message of the soups: the chef knows his seafoods. The oysters Rockefeller are fresh and moist, the spinach mild enough to let them shine and the hollandaise a lemony spark.

This same kitchen serves ribs that are dried out to the point of crunchiness and ladled with a sticky sweet-sour barbecue sauce, and veal oscar that is heavily breaded and soggy, topped with asparagus that is tough and dry but that frames the pearliest and sweetest lump crab meat under that very good hollandaise.

Again the point is made: stick with the seafoods. Try, for instance, a seafood sampler, at $10.50 a fine value that includes crisply breaded and very flavorful shrimp, a spicy and fresh- tasting crab cake that deftly misses being heavy, very good scallops and fried fish.

Which adds another important bit of information: the frying is also very competent. Catfish fillets, for instance (though admittedly not everybody would stand for catfish being filleted before frying), are lightly and crunchily coated with cornmeal and fried so that the fish is steamy and moist, the crust greaseless.

Not much is made of accompaniments, though the baked potato is cooked, thank goodness, without foil, and the cottage fries are fine. Otherwise there might be oversweet glazed carrots, an indifferent salad of iceberg lettuce and pedestrian dressings, uninteresting rolls, a short and high-priced wine list and desserts that taste no better than from a supermarket bakery except for a really jazzy show of bananas foster.

Where's the fried chicken? At brunch, at which for a mere $10 you can fill from a buffet of winey beef stew, shrimp creole (left too long on the steam table), scrambled eggs with bacon, sausage, saut,eed apples and some unfortunately pale hash brown potatoes. There are tables of picnic salads--potato, slaw and macaroni--that are dull stuff, some commercial-tasting quiches, super-sweet blintzes and that good juicy fried chicken. In addition to the buffet, you can have the chef make you an omelet to order, and he is good at it, especially if your order is for crab meat. Desserts are no more enticing than at dinner, though a huge silver bowl of fresh fruit salad makes a refreshing ending, and the coffee is terrific.

What makes Joplin's brunch special is not particularly the food in itself but its homey variety and the comfortable lingering. If at dinner the restaurant is "downtown," at brunch it is "down-home".