Internet security company Sourcefire Inc. said Wednesday that its net income fell by a third in the fourth quarter as sales and marketing costs grew more than three times as fast as revenue. Its first-quarter outlook fell short of analysts' forecasts and its shares fell.

Net income in the three months to Dec. 31 fell to $4.4 million, or 15 cents per share, from $6.7 million, or 23 cents per share, a year earlier.

Revenue rose 8 percent to $38 million from $35.3 million. But sales and marketing costs rose 30 percent to $14.3 million from $11 million.

Excluding stock options expenses and other one-time costs, adjusted earnings came to 17 cents per share, matching the expectations of analysts polled by FactSet. The sales figure also matched estimates.

For the first quarter of 2011, Sourcefire said it expects revenue from $28.7 million to $30.7 million and adjusted earnings of a penny to 3 cents per share after excluding stock-based compensation expenses of $3.1 million to $3.3 million and amortization of $1.1 million.

Those estimates were short of analysts' expectations for adjusted earnings of 6 cents per share on revenue of $30.5 million.

Sourcefire CEO John Burris said the introduction this year of an expanded security platform will significantly expand the number of customers it can go after.

Shares fell 45 cents, or 2 percent, to $22.25 in after-hours trading Wednesday after closing down $1.97, or 8 percent, at $22.70.