Investors poured $16.9 billion into exchange-traded funds in March, bringing the first-quarter total to $53.1 billion, according to Morningstar.

The fund researcher reports that ETFs gathered more than $50 billion in four of the past five quarters. U.S.-stock funds attracted $11.2 billion, the most of any asset class. However, Morningstar notes that the strong flows into U.S.-stock ETFs did not come at the expense of bonds, as bond ETFs gained $4.8 billion. For the month of March, commodities and international stock were the only asset classes to see outflows.�

Moreover, ETF inflows surpassed $50 billion in four of the last five quarters. Despite outflows of $8.1 billion from SPDR S&P 500 SPY, U.S.-stock ETFs were particularly popular in March, taking in $11.2 billion�the most of any asset class. But fixed-income ETFs had a strong month, too, collecting $4.8 billion.

Other findings of the report were:

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Commodities and international stock were the only asset classes that saw outflows in March. Investors pulled $4.1 billion from diversified emerging-markets ETFs in March, their first monthly outflow since May 2012.

At the firm level, Vanguard continued to gain market share, leading all ETF providers for the month and the quarter. Its market share increased to 19% from 17% one year ago.

Combined, iShares, State Street, and Vanguard hold 83% of ETF assets under management. Looking beyond the �big three,� WisdomTree, PIMCO, and Schwab have gained market share in the last year, while PowerShares, Van Eck, ProShares, and Deutsche Bank have lost ground.