GRINGOS, By Charles Portis Sinom and Schuster. 269 pp. $18.95

MEXICO, where dusty villages with a few drowsy hens pecking in dirt are edged by exotic jungles noisy with wild pigs, parrots and howler monkeys, where foliage hides the secret mysteries of ancient Mayan stone ruins. Mexico, where a chill may come from a bout of dengue fever or the discovery of a priceless jade carving in a junk shop, where you enter a quiet cave and two crazed commercial pot hunters suddenly leap out at you. "You never know what you'll run into in Mexico, John Knox in a guayabera shirt, or a rain of tadpoles in the desert, or a strangely empty plaza in the heart of a teeming city with not even a bird to be seen."

In the Mexico of Charles Portis's eminently readable Gringos we encounter just such a vivid and various landscape. Think back to the lazy hustle in a Mexican town that opens John Huston's "Treasure of the Sierra Madre," or the hot hopeless feeling of the setting in Tennessee Williams's "Night of the Iguana." That's how immediately and how flawlessly the author of True Grit draws us into his memorable gallery of expatriate gringos living in the backwater town of Merida in the Yucatan.

Our guide through Merida is that classic American loner often found narrating "drifter" movies. The Bogart or Glenn Ford type with a laconic blend of cynicism and sentiment: wry, self-contained, omni-competent and uncommitted. The outsider who just wants to be left alone, but ends up taking charge of everybody else's problems for them. In that low-key way, Jimmy Burns, ex-Marine grandson of a Louisiana minister, leads us through his story with a rambling tough-guy charm that never loses sight of the plot ahead, or ever forgets his L.C. Smith 12-gauge in a crunch. Smart and aware, Burns is even on to his own tricks -- his procrastination, his temper, his contradictions. He's a worry wart about his friends, but has a don't-fence-me-in attitude about his love life. To his alarm, women are always working on his moral improvement (offering him written lists of his shortcomings); now, one of them -- the appealing Louise "Born to Meddle" Kurle -- is planning his future with imperturbable relish.

Like many of his compatriots, Jimmy Burns has been stalled out in Mexico for years, consorting on the fringes of the law with con-men and hard luck cases, scratching a living by odd-jobs, hauling, scavenging (not only for junk but for runaways to turn in for the rewards). But Burns is also an expert on Indian antiquities (although he no longer digs them up illegally to sell), and spends time in intellectual talks with artists and scientists. He has a facetious contempt for many of them, particularly his countrymen down in Mexico to study its folk culture -- rich dabblers in ruins, ethnomusicologists, novelists working on grim modern allegories, curators on grants, dilettantes in primitiveness: "It was like some poet or intellectual going on about the beauties of baseball."

In fact, Jimmy knows lots of people of all types, and we meet quite a range of them -- from teams of American archaeologists on college digs to legless beggars in the town square; from a Mennonite farmer to a Mormon researcher ("a big strapping fellow with a clean beard and clear eyes, a walking tribute to Mormon dietary laws," who's out in the field alone, looking for evidence that Quetzalcoatl was actually Jesus). However small and quick the sketch, in Portis's skilled hand, the portraits of these individuals are dramatically sure. A trading post mongrel dog named Ramos is a more vibrant character than the humans in some minimalist fiction.

It is, of course, Burns's fellow gringos, the expatriates of Merida, who play the leading roles in his story. Like the losers and dreamers, crooks and drifters whom we've met hanging around foreign bars in Somerset Maugham's and Graham Greene's fiction, Portis's group of has-beens and would-be's huddle together in a limbo of memories and daydreams. Daily, they drop by the aptly named "In-Between Club" -- and there pass the time while time passes them by. Like the much-married Emmett who came to Mexico 30 years ago to cure an intestinal problem, none of them ever intended to stay. They are always giving each other farewell parties, and always never leaving -- until they die, or get deported, or drift elsewhere. These gringos were always something grand in the past: proud cranky old Frau Kobold was once on Fox Movietone News ("Bringing an Ancient Civilization to Light!"); someone else was in the Bowling Hall of Fame. And they always are about to become something grand in the future: Suarez, an old radical revolutionary who fled Spain in 1939 (after "eviscerating priests and burning down churches" with such gusto that in the comparison the Communists were "squeamish moderates"), is now impatiently awaiting the Armageddon of a pan-Hispanic uprising.

At their center is Doc Flandin, an arrogant, eccentric, professionally ignored archaeologist, who for decades has been writing the big book that will solve all old mysteries about all ancient Indian cultures, and decipher the Mayan hieroglyphics for good measure. Whether listening to Al Jolson records from his big carved canopied bed and wailing in self-pity that he is a "man literally murdered by the envy of cunning and hateful mice," whether bounding back to health to seduce an attractive female student, Flandin is one of those fictional stars who leap off the page into extravagant life whenever he appears.

Gringos is a quest novel, like True Grit; in this case, the journey is a search party Jimmy Burns leads to try to find Louise's presumed husband, Rudy Kurle. A gullible yuppie adventurer, in bush hat and safari jacket, Rudy is out after flying saucer landings -- a sucker, as Burns says, for images of barefooted astronauts and pre-Colombian Oldsmobiles. Now he's disappeared while off looking for the space dwarfs he's convinced once landed in Mexico.

Ancient ruins have lured a gathering of hippies to the same area, which is dangerously close to the river border patrolled by Guatemalan soldiers. Drawn there by "a buzz of magical words in the air" (a rumor started by a UFO newsletter), these New Age followers of hollow earth theories and pyramid power are flocking to the Inaccessible City of Dawn where they expect to witness the death of the sun and the annihilation of the world. Among them are spacey flower children (a girl with lightning bugs tied to her hair chants "Share the wonder, bring a friend") and reluctant pilgrims (a young man laments "There's not a single Pepsi left in town," and frets, "No way I'm walking into the jungle in my jogging shoes"). More ominously, on this rescue mission, Burns tangles with some hardened doper-types led by one Big Dan, a nasty ex-con biker who's decided he just may be the prophet El Mago.

Gringos can move from scenes as somnolent and dreamy as a siesta one minute, to show-downs as fast-paced and colorful as a mariachi band the next. At all times, in all tones, Portis is absolutely in charge, and the reader knows it, and what a rare, delightful relief it is be able to trust in that steady skill. Michael Malone is the author of the novels "Handling Sin," "Uncivil Seasons," "Time's Witness" and the forthcoming "Foolscap." He teaches at the University of Pennsylvania