By Ira Iosebashvili, James Ramage and Alexandra Wexler 

When Simon Mammon turned on his computer at 4:30 a.m. Thursday to check his trading portfolio, the owner of the Simon Sips cafe in Midtown Manhattan was just in time to see his bet on the Swiss franc SHYimplode.

The trade--a wager that the franc would weaken against the euro--had fluctuated by about $40 over the previous three weeks. That morning, it plunged by thousands of dollars after the Swiss National Bank announced it would no longer prevent the franc from rising, instantly sending the currency up 30% against the euro after three years of SHYstability.

"I was literally still in bed," said Mr. Mammon, 51 years old. "I'm trying to understand why all of a sudden it was down $2,500. When I looked away and looked back again it was five or six times that."

Mr. Mammon--who says he had juiced his bets by borrowing 20 times the amount of his original investment--sustained losses that day "well into the five figures." He was far from alone among retail, or individual, investors hit by the giant swing in the Swiss franc against the euro and dollar. Some of them had staked large sums using just tiny amounts of cash. It was a rare event that revived long-standing investor complaints about the retail foreign-exchange market.

Investors and analysts say the fallout is a major black eye for an industry that has struggled to escape a bad reputation--as a haven for unsound practices that is frequented by unscrupulous operators and novice traders who unwittingly amplify the risk of losing their stakes by using huge sums of borrowed money, or leverage.

"Historically, a lot of the FX brokers have been notoriously shady," said Larry Tabb, founder and chief executive of the Tabb Group, a research firm. The industry "has come a long way...in creating a reputable product."

Retail traders span the globe, from New York to Tuscaloosa, Ala., to Kuala Lumpur. Lively communities fill online message boards devoted to currency trading; one thread about the euro-dollar trade contains over 950,000 posts dating back 11 years.

Of the estimated four million retail foreign-exchange traders world-wide, the majority are in Europe and Asia, with only 150,000 in the U.S., according to data from Citigroup Inc. Most are male, with a median age of 35.

Just 30% achieve monthly positive returns, the data showed. About two-thirds of U.S. clients at FXCM Inc., the broker that was rescued Friday by a $300 million loan from Leucadia National Corp., lose money each quarter, according to filings.

Some experts believe the losses suffered by "mom and pop" investors will bring more scrutiny to this corner of the $5 trillion-a-day foreign-exchange market, where risk is high and rewards elusive.

Brokers allow traders to place bets of as much as 50 times their initial deposits in the U.S., quickly magnifying small currency moves into sizable gains or losses. In Europe and parts of Asia, leverage can reach 200 to 1, or higher.

"This is certainly going to spark regulator review, specifically regarding leverage," said Richard Repetto, an analyst at Sandler O'Neill + Partners.

A spokesman for the Commodity Futures Trading Commission, FXCM's primary regulator in the U.S., said the agency is reviewing the company's situation but declined to elaborate.

This past week, some firms in Russia and New Zealand also faced woes, stopping trading or going into insolvency. Many retail traders said they experienced problems processing trades and withdrawing their money at other retail brokerages, as well.

The news hit in the middle of the night for San Francisco-based Joshua Garrison, a former broker at FXCM. He now trades for his own firm, PoseidonFX, which offers research and trading tips to retail investors who are members of the site.

His firm had just, on Wednesday, made a bet that the euro would rise against the Swiss franc, after seeing the franc approach its cap of 1.20 to the euro. It was a wager they had made before without issue, he said.

Around 1:30 a.m. Pacific time, Mr. Garrison got a call from PoseidonFX's co-founder, Alex Boyd, another former FXCM broker. Mr. Boyd was in a panic after getting the alert that the trade was going haywire.

It took four minutes for them to confirm they had unwound the trade, compared with the typical seconds. "We had to keep checking and refreshing" the computer screen, Mr. Garrison said.

Their broker, CitiFX Pro, a unit of Citigroup, sent an email two hours later advising clients they might revise the executed trades to reflect worse pricing than they initially received. If that happens, their losses could grow to $50,000, Mr. Garrison says.

Duane Sloan, 49, a certified public accountant and former mayor of Wildwood, N.J., placed a bet through Toronto-based broker Oanda Corp. at 3:47 a.m. Thursday morning that the franc would rise against the yen. He was trading about 16,000 francs at 50-to-1 leverage.

Fighting insomnia, he said he had looked at the chart of the two currencies on his iPhone and reckoned the franc was due to tick higher. His account balance went from $361 to $2,703 in the space of an hour.

"To be completely honest, it was luck," Mr. Sloan said.

When he tried to close out the trade on his phone, he initially was unable to, because the broker had halted any transactions tied to the Swiss franc, he said. About 15 minutes later, "I could walk away and count myself lucky," Mr. Sloan said.

Tatyana Shumsky and Liz Moyer contributed to this article.

Write to Ira Iosebashvili at ira.iosebashvili@wsj.com and Alexandra Wexler at alexandra.wexler@wsj.com

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