As of the end of 2005, only 37 percent of Usana's associates had ever earned a commission, according to the company's latest figures. Among those who had been paid, the figures show, 87 percent didn't earn enough to cover the $116 they have to purchase or refer each month to qualify for commissions.

Usana says this kind of analysis misses the point. "The inherent goal isn't about coming in to, quote, break even," says Fred Cooper, the company's executive vice president of operations. Most associates are interested in purchasing the vitamins without commissions, Cooper says, and most distributors view what they can earn as a vitamin discount, not as a path to profits.

Minkow also says he is suspicious because of Usana's offshore ownership.

The company's chairman, Myron Wentz, renounced his U.S. citizenship in the mid-1990s and now claims citizenship in the Caribbean tax haven of St. Kitts and Nevis. He controls a 45 percent stake in Usana held by a company in the Isle of Man, in the Irish Sea, which in turn, is controlled by an entity registered in Liechtenstein.

Gilbert Fuller, Usana's chief financial officer, said Wentz renounced his U.S. citizenship because "he sees himself as a citizen of the world" and that the matter had nothing to do with the company.

Cole Chambers, of Broomfield, Colo., says he joined Usana looking for a "turn-key business." But after handing over $700 in sign-up and activation purchases, he couldn't resell anything or recruit anyone and eventually gave up. "I feel like the vitamins are so expensive." he said. Usana says people like Chambers can take advantage of its one-year, 90 percent refund for products returned by distributors who quit.

Usana charges associates $40 for a 28-day supply of its top-selling multivitamin, Usana Essentials, more than double the retail price of other premium vitamins. A premium vitamin from competitor GNC Corp., the Ultra Mega Green Multivitamin, sells for $17 for a 28-day supply.

Usana's price is also more than 20 times that of mass-market brands like Wyeth's Centrum. Minkow says Usana looks like a pyramid scheme to him — a fraud that works like a chain-letter, in which each new recruit has to find new members in order to profit, until the available pool of recruits dries up. Usana says it is doing nothing wrong. It says it doesn't force anybody to make the qualifying purchases.

In the past, the distinction between a pyramid scheme and a legitimate multilevel marketer has come down to a formula devised by the Federal Trade Commission.

That test looks at how much of sales are "retail," or sold to end users. Applying the formula to Usana, the company would escape classification as a pyramid scheme if at least 63 percent of its sales were "retail."

Deciding what a retail sale is can be tricky. Peter Vander Nat, the FTC economist who co-wrote a 2002 paper on the subject, says it depends on intent.

If people are buying because they want to use a company's products, those sales can count as "retail." If they are buying to stay in the game for future commissions, those sales wouldn't qualify, he said.

Usana says it doesn't keep track of distributors' sales to the public, but requires that at least 70 percent of its products be bought by "end consumers," which includes associates.

Minkow has sent a critical 500-page report on Usana to the Securities and Exchange Commission and the FBI. According to people familiar with the matter, the FBI plans to question the company. Neither the SEC nor FBI would comment.