Assets held in domestic exchange-traded funds increased last month, but couldn’t quite keep pace with the Standard & Poor’s 500.

According to a monthly survey by State Street Global Advisors, ETF assets rose 2.9% in February to $752 billion, but the S&P 500 increased 3.1% during the month.

State Street said in a report released Tuesday that all 13 ETF categories increased in February, but sector ETFs surged 4.95% to $72.9 billion.

As of Feb. 28, 856 ETFs were managed nationally by 31 asset management companies. The top three managers collective accounted for 83.8% of the U.S. listed ETF assets under management. As of Feb. 28, BlackRock [BLK], which bought the iShares family of funds from Barclays, managed 48.1% of assets, State Street [STT] had 23% share, and Vanguard 12.7%.

Despite this overwhelming majority, the share of total ETF assets held by these three managers declined 0.5% in February as smaller providers continued to gather assets.

An executive at BlackRock said that its ETFs continue to find new ways to gather assets. The company estimated that $2 billion of its iShares are in 401(k)s and that “in the next five years, it is conceivable that flows to ETFs within the 401(k) space could reach several billion dollars,” said Darek Wojnar, head of product research and strategy at BlackRock’s iShares ETF unit.

While ETFs held outside of tax-advantaged retirement plans offer just that feature, as well as intraday trading, the appeal of ETFs for 401(k) investors is their low fees, Wojnar said.