The world's largest mutual fund company pulled in more than 90% of the more than $15 billion that investors poured into U.S.-listed ETFs in Q1, adding fuel to the fire of fund managers and providers who fear Vanguard becoming too big.

According to new data from ETF.com Analytics, between those flows and the S&P 500 Index's 1.81% increase in the quarter, ETF assets rose to $1.736 trillion. That was up 2% from the end of the last year and up almost 20% from the end of 2013's first quarter, according to data compiled by ETF.com. The analysis also showed that Vanguard - with $2.4 trillion in assets under management and roughly half of those in indexed products - is close to moving up one notch to the No. 2 spot on ETF.com's ranking of issuer flows (see below). It has total ETF assets of $351 billion, compared with $379 billion for State Street Global Advisors, which is currently the second biggest ETF company in the world. According to ETF.com, Vanguard's continued success at marketing its "pure beta" capitalization-weighted index strategies will no doubt create additional pressures on an ETF industry that is already intensely focused on keeping costs low, ETF.com says.

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