SPDR S&P 500 ETF Trust (NYSE: SPY), $60.5 billion in short interest.
iShares Russell 2000 Index (NYSE: IWM), $20.9 billion in short interest.
PowerShares QQQ Trust, Series 1 (NASDAQ: QQQ), $19.7 billion in short interest.
iShares iBoxx $ High Yid Corp Bond (NYSE: HYG), $11 billion in short interest.
iShares MSCI Emerging Markets Indx (NYSE: EEM) 2.12%, $9.5 billion in short interest.
SPDR S&P Biotech (NYSE: XBI), $7.9 billion in short interest.
Short Percent Of Float: By far the most heavily shorted ETF is the SPY ETF, which tracks the S&P 500 and represents a simple bet against the U.S. stock market and/or a hedge against long positions in U.S. stocks. In the past 30 days, SPY short interest has increased by $4.8 billion. When it comes to short percent of float, however, the XBI Biotech ETF has the highest of the six ETFs mentioned above at 94%.