Assets flowed out of ETFs in May, the first time flows have reversed direction since August 2010, according to State Street Global Advisors, which just released its latest Snapshot report tracking ETF funds.

Flows into U.S. Bond ETFs grew with the Barclays U.S. Treasury Index shooting up 1.6% and the Barclays U.S. Aggregate Index rising 1.3%. The S&P 500 Index fell 1.1%, while the MSCI EAFE Index dropped 2.8%. Investors pulled money out of the S&P GSCI Index even though Gold remained flat.

Meanwhile, fixed income assets gained $5.3 billion, a sign that investors are moving into more conservative investments.

ETF flows declined $218 million in May. As of May 31, 2011, there were 1,074 ETFs with assets totaling $1.1 trillion managed by 35 ETF managers. The industry as a whole took a beating, with assets falling $22.7 billion in May, a decline of 2.0%.

Large Caps saw the greatest outflows, with a decline of $8 billion, a reversal from April when large caps saw $6.7 billion in inflows. Commodity ETFs also took a hit with $3.4 billion in outflows, after a strong April. International-Developed and Emerging Markets also fell, tumbling 2.8% and 2.6%, respectively. Domestic Large Cap, Mid Cap and Small Cap markets slid approximately 1%, while the US Aggregate Bond, the US Treasury Bond, and the US Corporate Bond markets increased approximately 1.5%.

The top three managers in the US ETF marketplace were: BlackRock, State Street, and Vanguard, which  account for approximately 83% of the US listed ETF market. The top three ETFs in terms of dollar volume traded for the month were the SPDR S&P 500 [SPY], iShares Russell 2000 [IWM], and iShares Silver Trust [SLV]. The top three ETFs in terms of assets for the month were the SPDR S&P 500 [SPY], SPDR Gold Shares [GLD], and Vanguard Emerging Markets [VWO].