They are relatively recent arrivals on the international trade scene, but the ports of South Florida have accomplished what the bay did for San Francisco - transformed an urban region into a crossroads of world commerce.

With exports and imports mushrooming in the past 15 years, South Florida's ports have grown to handle a vast majority of U.S. trade with the Caribbean Basin and a huge chunk of commerce with South America.And, while their cruise ships add a touch of glamour to the community, the economic impact of trade is far more significant. Trade spurs the demand for business, from freight forwarders to truckers, and nourishes small manufacturers whose production is tied to exports.

But as rapidly as South Florida's ports have become a major force on trade routes, other ports can overtake them. That's why port authorities are arguing that facilities must be constantly expanded to stay competitive. And the price tag for growth is rising.

The Port of Miami's wish list for new additions could run to three- quarters of a billion dollars.

"That's with a B," joked Carmen Lunetta, who has been Miami port director for 21 years and is the driving force behind expansion plans.

Miami, the country's eighth-busiest cargo port, is not alone in its recent growth. All of the region's harbors have snared a larger share of the Latin American and Caribbean trade.

Port Everglades in Broward, which ships about half the amount of container cargo as Miami but whose tonnage is higher overall because it receives nearly every drop of fuel used in South Florida, is wrapping up a $100 million cargo expansion program.

"We're coming off our best year in history, with $40 million in gross revenue and $16.5 million in net revenue," said James J. O'Brien, who took over as director in 1990.

The Port of Palm Beach, which ships Florida sugar to the rest of the United States, has just hired a Miami engineering company to draw up a growth plan.

But there are some surprises in what goes out of South Florida's ports. While the region exports some of the world's most sophisticated computer and medical equipment, some of its largest exports could more correctly be classified as garbage, valued because of recycling.

The largest single export from Port Everglades is scrap metal. Newsprint and paper products head the list of Miami's exports, part of it wastepaper headed for Europe for recycling.

At Palm Beach, exporters ship molasses, a byproduct of sugar refining, to Europe and Canada to be used in animal feed.

And nearly 20 percent of Miami's trade isn't even from the United States. Called transshipments, the merchandise from the Far East and Europe is only in Miami to be transferred from one cargo ship to another.

At the Port of Miami, Mr. Lunetta is betting on expanded trade because he is bullish on Latin America and the Caribbean, even more so with the ratification of the North American Free Trade Agreement. Cargo trade at the port doubled in the last five years.

The expected renewal of U.S. commercial relations with Cuba also has whetted his appetite.

"There is going to be more cargo than any of us can handle," he said.