Shares of Fortress Investment Group (FIG: chart) opened up 89% at $35 on its first day of trading Friday on the New York Stock Exchange.

On Thursday, the 34.3 million share offering sold for $18.50 each, at the high end of a $16.50 to $18.50 forecast range, raising $634 million.

The offering represents 8.5% stake in the company.

Goldman Sachs, Lehman Brothers, Banc of America, Citigroup, and Deutsche Bank managed the deal.

The underwriters have an option to purchase 5.1 million additional shares to cover overallotments.

Fortress net income increased to $192.7 million in 2005 from $40.3 million in 2003.

Fortress stock closed at $31 a share its first day of trading, up as much as 68% from the IPO price.