As markets continued to rise in April, assets held in U.S. exchange-traded funds increased for the third consecutive month.

According to monthly data from State Street Global Advisors, ETF assets rose 2.9% from a month earlier, to $830 billion.

ETFs' growth outpaced the Standard & Poor's 500 stock index's 1.5% advance in April.

Gains were almost evenly distributed across all 12 ETF categories, with six adding more than $2 billion of assets.

Small-cap ETFs had the largest increase, climbing $2.7 billion, while mid-cap ETFs picked up $1.7 billion. The data was released Monday.

At April 30, there were 888 ETFs managed by 31 investment managers nationally.

The top-three ETF managers collectively accounted for about 84% of the U.S.-listed ETF assets, down slightly from the end of last year.

BlackRock, which bought the iShares family of funds from Barclays, managed 47.7% of assets, State Street had a 22.9% share and Vanguard 13.1%.

Vanguard announced last week that it reduced commissions associated with its ETFs.

The Valley Forge, Pa., company said it would offer commission-free trading to its brokerage clients that use its lineup of 46 proprietary ETFs, $7 trades on stocks and $2 trades on nonproprietary ETFs.

Vanguard's move comes just a few months after Charles Schwab lowered commissions on online equity and non-Schwab ETF trades to $8.95.

Schwab offered commission-free trading when it launched its proprietary ETF family last year.

JPMorgan Chase & Co. is also in the competition, planning to introduce a pair of ETFs.

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