The price war among the largest exchange-traded fund companies is expected to heat up and even impact fixed income and emerging markets ETFs.

The biggest ETF providers—Vanguard, BlackRock, Schwab and State Street—are in a classic price war, Matt Hougan, editor-in-chief of IndexUniverse tells The Wall Street Journal.

“It’s a marketing race,” Hougan said. “You could see ETFs with zero expense ratios. As assets go up, prices will come down. I don’t know if it matters if it’s six basis points or zero. You’re starting to shave pennies. But it’s fund to watch.”

Scott Burns, Morningstar director of ETF research, agrees: “Things will get cheaper, both in terms of transaction costs and fees. It’s only a mater of time before it proliferates to international sector and fixed income funds.”

In October, for instance, Vanguard lowered the account minimum threshold for its reduced-fee Admiral class share from $100,000 to $10,000. While fees range depending on the fund, the rate on the flagship Vanguard 500 Index Fund fell from 18 basis points to seven basis points for every $100 invested. The rate on the Vanguard S&P 500 ETF fell even more, to six basis points.

By comparison, the BlackRock iShares S&P 500 Index and State Street Global Advisors SPDR S&P 500 ETF each charge nine basis points.

Earlier last year, Schwab cut the fees on six of its ETFs, evidently as a result of Vanguard’s challenge. The Schwab U.S. Broad Market ETF now charges six basis points.

By comparison, diversified U.S. stock index funds charge 62 basis points and actively managed diversified U.S. stock funds cost 1.38%.