Consumer complaints about airline service continued to rise in July, with Continental Airlines again leading the list of carriers that drew consumers' ire.

Consumers filed 5,995 complaints with the Department of Transportation for the month -- nearly six times the number of complaints about airline service a year ago and up from 5,155 complaints in June.

Continental and its sister airline, Eastern, which is also a subsidiary of Texas Air Corp., together accounted for nearly 40 percent of the complaints registered with DOT. Continental was the object of 1,541 complaints, or 45.9 per 100,000 passengers.

Continental challenged the statistical validity of the DOT figures, which are based on calls or letters to the federal agency. Continental spokesman Bruce Hicks said DOT used two-month-old data in computing the airlines' relative standing.

If DOT had used current numbers on total passengers to compute its figures on complaints per 100,000 passengers, the number for Continental would have gone down instead of up, he said.

"There's no question that the Continental service product has dramatically improved from what were real problems at the beginning of the year," he said. In February, Continental absorbed People Express, New York Air and Frontier Airlines.

Hicks said there were indications that some of the complaints against Continental and other airlines might have been generated by unions.

Eastern's standings in the negative ratings improved over last month's, but the improvement reflected increased complaints by passengers against other airlines rather than any improvement in Eastern's performance.

In June, there were 809 complaints (or 19.6 per 100,000 passengers) about Eastern's service, which made it the second-most-complained-about airline after Continental. In July, there were increased complaints about Eastern's service -- 849, or 19.92 per 100,000 passengers.

But other airlines fared worse. Northwest Airlines, which has been troubled by labor disputes, delayed flights and lost luggage, bumped Eastern off the second spot on the list with 948 complaints, or 28.11 per 100,000 passengers.

Complaints about service at Pan American World Airways and Trans World Airlines also increased beyond the level of complaints about Eastern. DOT received 287 complaints about Pan Am's service, or 26.4 per 100,000, and 471 complaints about TWA, or 22.23 per 100,000.

Delta Airlines, with 4.46 complaints per 100,000 passengers, received the lowest percentage of complaints of any major U.S. airline.

As usual, complaints about flight problems, including cancellations and delays, led the list of problems that prompted consumers to write. The second most frequently cited problem was baggage.

Increased complaints about consumer service have prompted calls for more regulation of the nation's air carriers and warnings by DOT to the airlines to do better. Continental has said throughout the summer that the DOT numbers and perceptions of service problems have not yet caught up with actual improvements in service.