BIRMINGHAM, Ala. -- Auto insurer Infinity Property and Casualty Corp. reported Thursday that its first-quarter earnings fell by more than half as investment gains declined and costs rose.

The company posted net earnings of $4.3 million, or 35 cents a share, in the January-March quarter. That was down 57.8 percent from $10.2 million, or 81 cents a share, in the first quarter of 2011.

On an operating basis, which excludes certain one-time gains and losses, Infinity earned $4.3 million, or 36 cents a share, down 40.3 percent from $7.2 million, or 57 cents a share from a year ago. Analysts expected earnings of 61 cents per share, according to FactSet.

Revenue increased by 13.9 percent to $287.4 million.

Earned premium rose to $277.1 million from $239 million. Investment income fell to $9.7 million from $10.3 million. Losses and adjustments rose by 20 percent to $214.8 million.

Infinity, based in Birmingham, Ala., reported a combined ratio of 99.9 percent. An insurer's combined ratio reflects its losses and expenses. A ratio above 100 means that for every dollar in premiums received, the company paid out more than a dollar to cover claims and other expenses. A number below 100 indicates profitability. Infinity reported a combined ratio of 98.4 percent for the first quarter of 2011.

The company also affirmed its guidance for earnings on an operating basis for 2012 of $2 to $2.50 a share.

During the first quarter, Infinity repurchased 22,800 shares at an average price of $55.97.

Infinity shares fell $2.50, or 4.6 percent, to close at $52.