Southwest Airlines Co.'s bottom line continued to soar, with profit jumping 13% in the latest quarter, as the airliner again benefited from cheap fuel and the modernization of its fleet.

Shares, which gained 19% over the last three months through Wednesday's close, rose 3.1% to $48.55 premarket.

The results, which topped Wall Street views, come as the company said its fuel and oil expenses dropped 2.9% in the quarter; the average cost of fuel was $1.78 a gallon, compared with $2 in the year-ago quarter.

The No. 4 U.S. airline by traffic said it expects a similar cost in the second quarter, compared with $2.02 a gallon in the second quarter of 2015.

Southwest and its U.S. airline peers have reported strong financial results in recent quarters, the result of slower growth, less competition and cheaper fuel.

Chief Executive Gary Kelly said the company expects "modest operating unit revenue growth in the second quarter," based on solid bookings and revenue trends so far in April.

In the quarter ended March 31, Southwest's unit revenue—the amount it takes in per seat flown a mile—rose 9.2% compared with the quarter a year ago. The metric is closely watched as a sign of demand and how well an airline is generating sales. Unit revenues have been declining across the industry because of relatively rapid expansions.

Dallas-based Southwest became the fourth-largest domestic carrier over the past four decades by flying exclusively within the continental U.S. But with dwindling growth opportunities at home, it has been expanding abroad.

Load factor rose to 80.5% from 80.1% a year earlier. Traffic shot up 9.9%.

Over all, Southwest posted a profit of $511 million, or 79 cents a share, up from $453 million or 66 cents a share a year earlier. Excluding special items, earnings on a per-share basis were 88 cents.

Revenue climbed 9.3% to $4.83 billion. Analysts surveyed by Thomson Reuters were looking for earnings of 84 cents a share on revenue of $4.81 billion.

Write to Joshua Jamerson at joshua.jamerson@wsj.com

(END) Dow Jones Newswires

April 21, 2016 07:35 ET (11:35 GMT)

Copyright (c) 2016 Dow Jones & Company, Inc.
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