The Vanguard Group's exchange-traded fund lineup more than doubled in terms of assets under management last year as the mutual fund titan rolled out 18 new ETFs, CBS MarketWatch reports.

Assets under management in Vanguard's proprietary VIPER ETFs grew to $5.9 billion from $2.6 billion since the end of 2003, primarily due to the fact that the VIPERs family grew to 20 from two investments during the same period.

Vanguard senior officials said the VIPERs, which now offer a broad spectrum of investment strategies ranging from small-cap to health care, are a natural complement to the firm's flagship index fund offerings.

Although the ETF business represents a small part of Vanguard's assets, company officials intend to capitalize on successful launches last year by introducing additional equity VIPERs by midyear that will focus on emerging-markets, Europe and the Pacific.

Meanwhile, the firm's flagship index fund, The Vanguard 500 Index Fund, ballooned to $107 billion last year and emerged as the world's largest mutual fund.

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