Driven by the success of exchange-traded fund provider BlackRock, ETFs appear to be gaining notable market share in 401(k) plans, and while their use so far has primarily been among small companies, once the trend catches on among larger employers, ETFs’ popularity in 401(k) plans could become quite reports.

Of the $2.7 trillion Americans have in 401(k) plans, $2 trillion is in mutual funds, according to the Profit Sharing/401(k) Council of America. The remaining $700 billion is in a number of other investment choices, including collective investment trusts, separately managed accounts and ETFs.

BlackRock [BLK] estimates $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.