NEARLY any new or used book store will carry some science fiction; and even a Waldenbooks or a Crown outlet will stock a fair number of current titles, remainders and classics. But the Washington area also hosts several specialty bookshops with large stocks of fantasy, science fiction and related matter (comics, role-playing games, Star Trek posters).

DISTRICT OF COLUMBIA

MOONSTONE BOOKCELLARS -- 2145 Pennsylvania Avenue NW. 659-2600. Washington's best-known shop for new, in-print sf, Moonstone also handles that other popular genre, mysteries. Literally in a cellar, Moonstone crowds a vast number of books into its space: paperbacks, best-sellers, a few remainders, magazines and small press books (this last a prime source of sf criticism and illustration). If it's in print, owner Phil Grossfield has it, or he'll order it.

T CIRCLE -- 1340 Connecticut Avenue NW. 785-1133. The other Olsson's branches -- in Georgetown and Alexandria -- also carry good selections of sf, but the Dupont circle store boasts the on-site expertise of Stephen Brown. As a writer, critic and fan, Brown has an extensive knowledge of fantasy and sf (it was Brown who broke the story that Richard Bachman is Stephen King). His advice on reading sf: "Beware of the old guard which is pretty much written out." Instead of Arthur C. Clarke and Robert Heinlein, Brown recommends Philip K. Dick, William Gibson and Greg Bear, among others.

CHAOS UNLIMITED -- 3512 Connecticut Avenue NW. 244-2710. Run by Nelson Freck and Katherine Tuttle, this is Washington's best in-town source for used and out-of-print sf. The shop, located on the second-floor of a building near the Cleveland Park Metro stop, specializes in mysteries as well as sf. Asimov in paperback, Anne McCaffrey in inexpensive book club reprints, Roger Zelazny in signed editions from specialty publisher Underwood-Miller -- all kinds of books will be found here, with prices ranging from a dollar to a hundred dollars (or more). Chaos also issues a mail-order catalogue for those preferring to shop by post.

MARYLAND

GEPPI'S COMIC WORLD -- 8317 Fenton Street, Silver Spring. 588-2545. One of the nation's biggest comic book distributors, Geppi's also carries a lot of science fiction (as well as Transformer robots, T-shirts and movie posters). Racks of new books fill up the basement of the Silver Spring store; one wall offers a good range of used paperbacks and the odd remainder; and glass cases display limited editions from Underwood-Miller, Phantasia Press and other fine publishers. A smaller store is in the Crystal City Underground.

IMAGINATION BOOKS -- 946 Sligo Avenue, Silver Spring. 589- 2223. Less a bookshop than a group of bookshops, Imagination hosts dealers in a variety of genres: military books, romances, general literature, etc. Wayne Morgan specializes in science fiction and black literature; his stock is small, well-chosen and mostly paperback. Jim Gscheidle handles all kinds of reading matter, but his selection of sf includes hard and soft-covered books reasonably priced.

BARBARIAN BOOKS -- 11254 Triangle Lane, Wheaton. 946- 4184). Like Geppi's, Barbarian focuses on a mix of comics, gaming material and sf. Crowded and chaotic inside, the shop has a lot of good material here, though you may have a little trouble finding it. New and used.

ATTIC BOOKS -- 357 Main Street, Laurel. 725-3725. This excellent small shop moved last year from Wheaton to Laurel. Very nice used, scarce and out-of-print sf material.

ROBERT A. MADLE: SF/FANTASY BOOKS -- 4406 Bestor Drive, Rockville. 460-4712. By appointment only and mail order, Madle is the area's preeminent new and used sf book dealer. He stocks virtually everything from the small presses, carries large runs of sf pulp magazines (Amazing, Thrilling Wonder Stories, Weird Tales), and offers regular catalogues listing his treasures. Any serious reader or collector will want to be on his mailing list. A few first-edition highlights from his most recent catalogue may tantalize: H.P. Lovecraft's first book, The Shunned House (one of 300 copies), can be yours for $1,800; a later Lovecraft collection, The Outsider and Others (1939), goes for $525; certain novels by Philip K. Dick and Robert Heinlein regularly sell in the $300 range; and even a 1978 book, Stephen King's The Stand, is listed at $200. Of course, most of Madle's books sell for a lot less ($5 to $20).

THE BOOK ALCOVE -- Loehman's Plaza Rockville, 5210 Randolph Road. 770-5590. This general bookshop carries an exceptionally good selection of used sf paperbacks and hardbacks. A few pricey items, but otherwise a good place to pick up something to read.

DREAM WIZARDS -- 84 Halpine Court, Rockville. 881-3530. This shop ranges from science fiction and fantasy to include books on magic, mysticism, fantasy games, surrealism and related items.

VIRGINIA

TOMORROW BOOKS -- 1304 King Street, Alexandria. 548-5030. A small friendly shop, crammed with books, comics and -- on the weekends -- kids.

HOLE IN THE WALL BOOKS -- 905 W. Broad Street, Falls Church. 536-2511. This shop recalls both Wheaton's Barbarian Books and D.C.'s Chaos Unlimited: It is overflowing with material, managed by knowledgeable people, and all in all a first-rate source for new or used comics and sf.