Investors poured an estimated $4.2 billion into U.S.-listed exchange-traded funds and notes during the month of May, bringing total year-to-date inflows into exchange-traded products to $63.1 billion, according to just-released data from the ETF Industry Association. That’s up 21% from the $49.68 billion ETFs and ETNs received in inflows during the first five months in 2011 and more than twice the $27.57 billion they received in 2010. 

For the month of May, fixed income-based ETFs and ETNs led the charge, taking in $8.9 billion in inflows. International equity and commodity exchange-traded products, on the other hand, posted huge outflows, losing $3.27 billion and $1.51 billion, respectively.

The top recipient of the estimated $4.2 billion recent monthly inflow was Vanguard Barclays Total Bond, which took in $1.23 billion for May. The next two recipients were Vanguard S&P 500 and iShares iBoxx Inv Grade Corp Bond, which saw inflows of $644 million and $614 million, respectively.

Assets in U.S.-listed ETFs and ETNs totaled approximately $1.4 trillion at the end of May, up 2% from May 2011, when assets totaled $1.11 trillion. The bulk of the assets (71%) were held in equity- and REIT-based ETFs, which totaled $807 billion, according to the data.

The ETF Association tallied a total of 1,465 ETFs and ETNs at the end of May, a 17% increase from the 1,254 products listed a year ago.