=DJ GETTING PERSONAL: Hot Natural Gas ETF Faces Trading Snag By Ian Salisbury A DOW JONES NEWSWIRES COLUMN NEW YORK (Dow Jones)--One of the hottest investments on Wall Street may have gotten too big for its own good. With investors betting on rising gas prices, assets in the United States Natural Gas Fund (UNG) recently swelled to almost $3.7 billion from about $670 million in February, even sparking fears it could be disrupting the futures market. Now the popular exchange-traded fund is days away from another potential problem: Funds that hold commodities typically face stiff restrictions on the number of shares they can issue to meet investor demand, and United States Natural Gas is running out fast. Securities and Exchange Commission filings show managers want to increase the number of shares available nearly 10-fold. But such requests can take weeks and there is no telling when the SEC will act. If the fund can't issue enough shares to meet investor demand, its shares could begin trading at prices higher than the underlying value of their holdings, breaking a key promise ETFs make to investors and possibly influencing prices in the natural-gas futures markets. Exchange-traded funds resemble open-end mutual funds but trade on an exchange like a stock. The issue is the latest in a string of surprises that have trailed more exotic types of ETF, especially ones that hold commodities. It also highlights the friction between ETF firms and regulators, such as the SEC, whom fund company executives often complain misunderstand and unnecessarily delay many requests regarding ETFs. SEC spokesman John Heine declined to comment directly on the case of the natural gas fund but said the Commission is sensitive to issuers' time constraints. The SEC's position involves trade-offs, says Michael Berenson, a securities lawyer at Morgan Lews in Washington. If the regulator ignores the ETF's predicament, it could risk harming the fund's shareholders. But rushing might not be fair to other ETF firms with business before the SEC. "It puts everyone else back in the queue," he says. Commodity ETFs have proved wildly popular with investors but have had their share of growing pains. Investors were initially dismayed to discover some funds, including ones designed around oil and natural gas, match returns of futures contracts, not spot prices, which can be substantially different. The funds also became embroiled in last summer's controversy over whether speculators were creating an energy price bubble and by other worries about whether large investors have taken advantage of the funds' need to roll holdings each month, profiting at fund-holders' expense. ETFs accommodate investor demand by essentially creating new fund shares whenever investors want to buy them. But commodity funds, with a unique legal structure, don't have the same flexibility in this regard as stock ETFs. Anticipating a spike in demand earlier this year, managers of U.S. Natural Gas Fund filed with the SEC to boost the number of shares the fund kept available by about 300 million in late March. By early May, the SEC had yet to act, and the company made another filing warning it had only about 8 million shares in reserve and could soon run out. The original request was granted a few days later, but only after the fund had been unable to meet investor demand for about two days, nearly imperiling its operations.