Principal Financial Group Inc.’s first-quarter net income fell as unstable equity markets led to an investment loss and premium income dropped, the company said. Operating earnings rose by $4.5 million.

The company reported a net income available to common stockholders of $174.2 million, or 67 cents per share, for the quarter ended March 31, compared with $257.1 million, or 95 cents per share, for the three months ended March 31, 2007.

Principal reported operating earnings of $241.3 million for the first quarter of 2008, compared with $236.8 million for the same quarter of the previous year. Operating earnings per diluted share (EPS) for first-quarter 2008 were 92 cents compared with 87 cents for the same period in 2007. Operating revenues for first quarter 2008 were $2,639 million compared to $2,623.7 million for the same period last year.

“In light of a highly difficult operating environment, we view 6 percent growth in EPS from first quarter 2007 as a very solid result, one that demonstrates the benefit of a diverse portfolio of businesses,” said Larry Zimpleman, president and CEO of Principal, in a release.

The company also reported additional highlights for first quarter 2008, including total company assets under management of $304.2 billion as of March 31, an increase of 13 percent from a year ago (including $86.6 billion of third-party assets for Principal Global Investors), record full-service accumulation sales of $3.5 billion, an increase of 57 percent compared to the same period a year ago, and record full-service accumulation net cash flows of $3.0 billion, an increase of 126 percent.