U.S. Natural Gas ( UNG) was the ETF that gained the most investor capital in June, a whopping $1.7 billion. The next closest competitors for investor dollars were iShares Barclays ( TIPS), with $0.9 billion; Vanguard MSCI Emerging Markets ( VWO), with $0.8 billion; SPDR Retail ( XRT), with $0.7 billion; and iShares S&P 500 ( IVV) Index, with $0.6 billion. Overall, investors added $12.1 billion to ETFs in June, meaning UNG accounted for 14% of new money. The flow into UNG was even more impressive in percentage terms for the fund, as assets under management swelled from $2.2 billion to $3.7 billion in a month when UNG declined 5%. It also proves that investor interest isn't always enough to push prices higher. UNG fell on the first two trading days of July and is down again today, setting it back more than 9% from the June close. Natural gas traded in sympathy with oil over the past few days as investors question the economic recovery. Weak jobs numbers on Thursday were a blow to many who had believed the "green shoots" story, and a psychological correction in the commodity and stock markets is quite possible, even without deteriorating fundamentals to help it along. I've covered UNG several times. Most recently, I looked at the drawbacks to the UNG fund and discussed a potential ETF from the same issuer, U.S. Commodity Funds, designed to deal with the the problem of contango. Despite the drawbacks of UNG and a similar ETN, iPath Natural Gas ( GAZ), investors want access to the commodity, and these are the only two ways of gaining direct exposure. GAZ had a similarly impressive rise in assets in percentage terms, up $61 million in June, lifting AUM to $105 million.