This season, Americans are finding sweet, juicy clementines stacked higher than ever in supermarkets and warehouse stores, destined for Christmas stockings and the children's lunch boxes. Juan and Javier Arnal of Spain are two of the reasons why.

Juan is a 72-year-old grower and exporter, and Javier is his 38-year-old nephew. And the story of how they have helped propel Spanish clementines onto the American market has as much to do with learning to play professionally in the global marketplace as it does with the instant appeal of the fruit itself.

Spain, which holds nearly a 90 percent share of the world's clementine export market, sends most of the diminutive citrus fruit, a variety of mandarin orange, to other European countries. In 1991, only 5,180 tons went to the United States. Seven years later, that figure hit 50,706 tons. That is still only 4 percent of Spain's clementine output, but it has hit the United States like an orange tsunami. Supermarket chains, Kmart, Wal-Mart and big-box stores from Silver Spring to Seattle are stocking the fruit. Of course, the numbers pale beside the 1.5 million tons of navel oranges California is expected to produce this season. Nonetheless, in many areas of the country, the distinctive five-pound crates--which this year are selling for $4 to $9--have gone from gift item to winter staple in just a few years.

Much of America's clementine explosion can be attributed to Nulexport, a cooperative of more than 1,000 small farmers in Nules, Spain, a town of 16,000 on the Mediterranean coast, about 35 miles north of Valencia. Nulexport is also the single biggest exporter of clementines into the United States, and Juan Arnal is its general manager, a strong man with a white mustache who still farms citrus on some of the same land his grandfather did 100 years ago.

Arnal's father, too, was a grower, packer and shipper, and the family business, which in good years produced about 20,000 tons a year, was passed to Juan and his three siblings. But two decades ago there was a disagreement among them, and Juan wanted out. That's when Nulexport, a fledgling citrus cooperative of 37 farmers selling only 800 tons of citrus a year, approached him. "They were poorly managed," Arnal said in a telephone interview, with nephew Javier translating. They wanted him to shake things up.

With experience in all aspects of the citrus business--growing, picking, packing, selling--Juan Arnal accepted the challenge. But rather than marketing what the farmers produced, he focused on producing what the European market wanted. He persuaded the growers to switch from satsumas, a type of tangerine, to a large, juicy, durable variety of clementine called the Clemenules. He automated the sorting and packing operation. He channeled the product through large distributors.

By the time Javier Arnal joined Nulexport about 11 years ago, production of Spanish clementines was strong. But there wasn't much more growth in the European market. The younger Arnal wanted to look elsewhere.

"I have in my blood that I don't want to be in Spain in the packing house," said Javier, who now oversees shipments of the clementines at Holt Terminal in Camden, N.J., reporting sometimes three times a day by phone to his workaholic Uncle Juan, who puts in 12- to 14-hour days back in Spain.

"The only market I found that would work was the U.S. and Canada," Javier said. The Clemenules could withstand the trip across the Atlantic and the time it takes to get into supermarkets, he figured. And both countries had consumers who could pay the added costs for transportation.

Already, clementines were being exported in a small way to the United States from Spain and Morocco. They'd go mostly into New York, sold as loose produce at corner greengrocers. But there was never a continuity of supply, and the quality wasn't consistent, according to Luke Sears, president of LGS Specialty Sales Ltd., a Bronx importer with whom Javier Arnal joined forces. The clementines were packed in a refrigerated compartment on a container ship with "cars or stereos or whatever, and the monitoring of the temperature wouldn't be very good," Sears said.

Now, LGS, which also distributes clementines from four other Spanish cooperatives, charters its own vessels, bringing in two boatloads a week, filled exclusively with clementines--a million crates per ship. And LGS is the largest importer of Spanish clementines, with 50 percent of the Spanish exports.

But before all that would happen, Nulexport and other cooperatives in Spain had a lot to learn. "In order to export to this country and maintain the proper quality, you have to heavily invest in technology," said Sears.

Javier Arnal was no stranger to technology, given the investment his uncle had already made in supplying the European market. So Javier flew to California, where he bought packing machines from Sunkist and machines to "size" the fruit electronically from a company in Fresno.

There were other challenges. As it does with other imported produce, the U.S. Department of Agriculture has requirements: All clementines from Spain must be cold-treated--meaning refrigerated at specified temperatures for specified times--to kill off any Mediterranean fruit flies, unwanted intruders for American citrus growers. Better refrigeration units were needed to ensure proper temperatures, both at the packing plant and on the vessels.

Then there were the U.S.-approved fertilizers, pesticides and insecticides, all of them new to growers who had been producing for the European market.

With a basic infrastructure in place by the early 1990s, Sears and Arnal began to target chain stores rather than corner greengrocers. Other smaller exporters were getting more savvy at the same time, too, and made contacts with American importers who also sold directly to chains. "The exporters in Spain did their research," said Jim Gatter, director of sales and marketing for Sunny Valley, a Glassboro, N.J., importer that sells clementines directly to supermarkets, including Fresh Fields.

The product took off, far exceeding expectations. More importers and exporters jumped on the bandwagon, and the distribution and marketing got increasingly organized. With this early taste of success, the Spanish government began to commit marketing dollars--in the last four years a couple million dollars a year in radio spots and promotional events for the past four years. Success begat success.

And no wonder. Clementines have many of the qualities coveted by snackaholic Americans, save for being fried. They're portable, seedless and simple to peel. Even cookie-craving children like them. "They're very sweet, like eating candy," said Charlie Lester, Giant Food's director of produce purchasing, who added that the chain's sales of clementines have increased fourfold in just four or five years.

The packaging sets them apart. "In my mind what sells the clementine is the wooden box," said Russ Hodson, Spanish import coordinator for DNE World Fruit Sales, a Fort Pierce, Fla., importer. The box "separates [the clementine] from every other item in the produce department."

Clementines have long been packaged in wooden boxes for the European holiday gift market, according to Juan Calvo, president of Ibertrade, the Manhattan-based importer that was one of the first to introduce clementines and which has a 20 percent share of the market, second only to LGS. (According to Foods of Spain, a promotion agency for the Spanish government, 13 importers share the remaining 30 percent.)

But the crate is significant in the United States. In Europe, clementines are sold in mesh bags throughout the non-holiday season, according to Javier Arnal. But in North America, "bags mean second-class fruit," he said--quantity rather than quality. Plus, crates are easier for chain stores to handle, said Arnal.

Supermarket mergers have also made it easier for LGS and other importers to introduce clementines to areas outside the Northeast, where the fruit got its foothold and where sales are still strongest. "Retailers who have divisions in the Northeast are quicker to sell clementines into other parts of the country," said Sears of LGS. They have a track record.

Selling clementines to some of this country's citrus-producing states hasn't been so easy--for obvious reasons. "Retailers want to show loyalty to native products," Sears said.

Not surprisingly, California and Florida have taken notice of the rising popularity of the fruit, which grows best in areas with warm days and cool nights. Florida's tropical climate isn't as conducive, but California took the plunge.

"We recognized that we were competing with a top-quality product from Spain," said Joel Nelsen, president of California Citrus Mutual, a growers group. So California planted a small number of clementine trees that just came to market maturity last year, said Nelsen. Another several thousand acres will come into production by 2003.

Javier Arnal isn't too concerned about a major threat from California, given that it's doubtful a significant portion of growers will convert from navel oranges to clementines.

But he doesn't underestimate California's agricultural potential. "Who knows?" he said. "In 10, 15 years, we may be out of the market."