Vanguard's exchange-traded fund assets more than doubled last year as the mutual fund titan rolled out 18 new ETFs. 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 VIPER 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 funds. Although the ETFs represent a small part of Vanguard's assets, the firm intends to introduce additional equity VIPERs by midyear that will focus on emerging-markets, Europe and the Pacific.

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