Exchange-traded funds (ETFs) are on a torrid asset-gathering pace, one that remained in place in April as U.S.-listed ETFs added over $36 billion in new assets. That brought the assets under management tally for exchange-traded products (ETPs) listed in the U.S., the world's largest ETF market, to a record $2.865 trillion, according ETFGI, a London-based ETF research firm. With U.S. stocks notching a series of record highs last month, equity ETFs remained popular with investors. Eight of the top ten asset-gathering ETFs in April were equity funds, with the other two members of the top ten being a bond fund and a gold ETF. (See also: ETFs Poised for $1.1 Trillion Asset Surge.)

"In April 2017, ETFs/ETPs listed in the U.S. gathered $36.09 billion, a record amount of net inflows for  April, marking the 14th consecutive month of net inflows," said ETFGI. "Equity ETFs/ETPs gathered the largest net inflows with $26.41 billion, followed by fixed income ETFs/ETPs with $8.83 billion, while commodity ETFs/ETPs experienced net outflows of $889 million." (See also: Maybe Some Hope for Commodities.)

As U.S. equities climbed last month, investors responded by pouring new money into popular ETFs such as the iShares Core S&P 500 ETF (IVV) and the Vanguard S&P 500 ETF (VOO). However, plenty of enthusiasm remains for international ETFs, both of the developed and emerging markets varieties. The iShares Core MSCI EAFE ETF (IEFA) continues packing on new assets, as highlighted by April inflows of $2.13 billion. One of IEFA's primary competitors among low-cost, ex-U.S. developed markets ETFs, the Vanguard FTSE Developed Markets ETF (VEA), saw April inflows of $1.55 billion. Led by the iShares Core MSCI Emerging Markets ETF (IEMG), emerging markets ETFs also continued adding new assets in April. (See also: Why These Country ETFs Are Soaring.)

Underscoring the point that low expense ratios remain a primary selling point, six of April's top ten asset-gathering ETFs, including the likes of IEFA, IVV and VEA, can be considered low-fee products. In fact, many of the top asset-gathering ETFs last month were among the least expensive products in their respective asset classes.

Through the first four months of this year, U.S.-listed ETFs have added nearly $170 billion in new assets, well ahead of the $45.3 billion added in the first four months 2016. "iShares gathered the largest net ETF/ETP inflows in April with $24.21 billion, followed by Vanguard with $9.93 billion and Schwab ETFs with $2.53 billion net inflows," said ETFGI. (See also: Guide to ETF Providers.)

Want to learn how to invest?

Get a free 10 week email series that will teach you how to start investing.

Delivered twice a week, straight to your inbox.