That wasn't enough for Edwards, who began looking to expand his business. Unfortunately, New York was already crowded with crack dealers; outside the city, however, lay plenty of virgin territory. In Washington, Baltimore and Philadelphia, for instance, crack was just beginning to catch on. Enterprising local dealers would travel to New York, buy a few ounces of cocaine, return home, convert it into crack, and sell the product for three or four times the New York street price. In the fall of 1986, Edwards traveled to Washington and set up shop; by the following spring his lieutenants had established thriving businesses in Philadelphia and Baltimore as well. At its peak, Edwards's organization, known as the Rankers, employed 50 workers and made up to $100,000 a day.

The glory days did not last. Edwards - nicknamed ''Uzi'' for his taste in weapons -was pathologically violent. People who crossed him were pistol-whipped, beaten with baseball bats, shot in the legs. One 16-year-old worker, suspected of cheating, was beaten unconscious with bats, scalded with boiling water, and suspended by a chain from the ceiling until he died. Eventually, the police caught up with Edwards, and in July a Brooklyn jury convicted him on 42 counts of murder, assault, kidnapping and drug dealing. Edwards is now awaiting sentencing. The Rankers have disintegrated.

But there are 40 other groups just like the Rankers, running crack out of New York and Miami to points across the country. Posses, they're called, after their members' affection for American westerns (and the guns used in them). Most, like the Rankers, took shape as gangs during the 1980 Jamaican election, then fled to the United States and regrouped. Here, their 10,000 to 20,000 members, organized in posses with as few as 25 members and as many as several hundred, keep incessantly on the move, slipping in and out of the many Jamaican communities scattered across the country. To maintain loyalty, each posse generally restricts membership to the residents of a particular neighborhood in Kingston. Posse members travel with fake IDs, making it tough for policemen to identify them. Sometimes, as a cover, they attach themselves to reggae groups touring the country. Today, Jamaicans are believed to control 35 percent to 40 percent of the nation's crack network.

''They're very good businessmen,'' says John A. O'Brien, an agent with the Bureau of Alcohol, Tobacco and Firearms (B.A.T.F.), the Federal agency that most closely monitors the posses. ''They follow the law of supply and demand. When they see that a vial of crack selling for $5 in New York will get $15 in Kansas City, they'll move in.'' New York is their ''training school,'' O'Brien says, ''like going to Wharton. They'll take a guy doing a good job in Harlem and send him to open an office in the Midwest.'' On his arrival in the new area, the posse sales rep will rent a motel room and conduct a market survey of sorts to determine the most lucrative spot in town. Then he'll rent an apartment or, better yet, get a single female to lend him one in return for crack.

When asked how the posses move the drug from city to city, Bill McMullan, the assistant special agent in charge of the D.E.A.'s Washington office, jokingly cites the title of a recent movie: ''Planes, Trains and Automobiles.'' Amtrak, Greyhound, commercial airlines, Federal Express, U.P.S. - the posses use them all, regularly.

To carry cocaine on commercial flights, the Jamaicans tend to recruit overweight women able to conceal one- or two-pound packages on their person. Also popular are rental cars, preferably Volvos, sent over the nation's highways, preferably Interstates. ''When I see some of the places the posses are operating, I can't find any other explanation than the presence of a nearby Interstate,'' says Stephen Higgins, the B.A.T.F.'s director in Washington.

In deciding where to strike, the Jamaicans generally follow the path of least resistance. Cities with well-organized criminal groups, such as Newark, St. Louis and Chicago, tend to get bypassed. At first glance, Chicago would seem to be an ideal posse target. It is a major transportation hub, has a vast inner-city population and offers block upon block of public housing projects, a favorite crack target. But Chicago also has plenty of established homegrown gangs doing a brisk business in cocaine and heroin. Intent on protecting their trade, they have worked determinedly to keep outside traffickers from entering. ''When crack first appeared, some groups did try to come here and stake out some territory, but they quickly left,' '' says Vincent Lang, chairman of the Chicago Housing Authority. ''They wanted to remain alive.'' Today Chicago is awash in powder cocaine, but crack is very hard to find. Perhaps wary of the anarchic market forces crack has unleashed elsewhere, local dealers have opted out of selling it themselves.