No-frills Southwest Airlines Co again topped rivals in customer service while premium-paying business travelers are the least satisfied with U.S. carriers overall, a new survey showed on Tuesday.

Airlines maintained their low overall standing among a variety of industries included in the American Customer Satisfaction Index, which is compiled by the University of Michigan's Ross School of Business.

There's been a bubbling discontent for airlines for some time but the situation has worsened slightly from a year ago, said ACSI managing director David VanAmburg.

Passenger satisfaction with airlines dropped by 1.5 percent to a score of 65 on ACSI's 100-point scale. Scores have generally hovered in the mid-to-low 60s for the past decade.

Travelers cited poor service, higher prices and fees for baggage and other services as the main causes of their discontent.

Airlines have raised fares and fees to counter soaring fuel costs and preserve a fragile financial recovery.

Southwest continues to outperform rivals with consumers, according to the survey of 2,000 consumers.

The low-fare carrier posted an ACSI score of 81 in part because it has not taken anything away from customers and then offered it back for a fee, VanAmburg said.

Southwest heavily promotes its policy of not charging for bags.

Among Southwest's main rivals, Continental scored 64, American 63, United and US Airways tied at 61 and Delta Air Lines dropped to 56 on the ACSI scale. American Airlines was unchanged at 63.

All other carriers, which include smaller lower-fare and service-oriented businesses, posted a 76, a 1.3 percent improvement.

A red flag for airlines in the latest survey is the dissatisfaction of business travelers, who the industry courts relentlessly and depends on for its highest fares.

We're seeing a greater discontent among business travelers simply because they are putting themselves out there more to be let down by the airlines or an experience, said VanAmburg.

Recent mergers, known to have a detrimental affect on satisfaction and geared specifically to attract more business travel, pose added pressure.

Delta plunged to the bottom of all the airlines for customer satisfaction one year after completing its acquisition of Northwest, ACSI reported.

The fate of United, which absorbed Continental last year, and Southwest, which acquired AirTran, remained uncertain, it said.

(Reporting by Lauren Keiper; Editing by John Crawley)