December brought no cheer at Continental ( CAL), at least where revenue was concerned.

The airline said mainline unit revenue, as measured in revenue per available seat mile, or RASM, fell between 4.5% and 5.5% from a year earlier. Mainline statistics exclude flights by the airline's regional carriers. Consolidated RASM, which includes those flights, fell between 4.0% and 5.0%.

Industry watchers pay close attention to Continental's monthly performance report because the airline is the only major U.S. carrier to disclose monthly unit revenue data. Airline revenues have been sagging because of a glut of capacity and fierce price competition.

Continental said both figures were negatively impacted by about 2 percentage points by the movement of Thanksgiving return traffic into November this year. They were also negatively impacted by another 2 points by the timing of Christmas this year, which caused return traffic to occur in January instead of December, the airline said.

The sharp decline in Continental's unit revenues came even as the carrier reported increased passenger traffic and filled a record number of seats on its planes. Mainline traffic, a measure of passenger demand expressed in revenue passenger miles, or RPMs, increased 6.5% year over year to 5.5 billion, while mainline capacity, expressed in available seat miles, or ASMs, rose 6.1% to 7.1 billion.

Consolidated RPMs jumped 7.8% to 6.2 billion on a capacity increase of 7.5% to 8.1 billion.

Continental's mainline load factor, which measures the percentage of seats filled on its planes, was 77.3%, 0.3 points above December 2003, and its consolidated load factor was 76.5%, up 0.2 points from a year earlier. Both were records for December.

The report could disappoint investors. Merrill Lynch analyst Michael Linenberg had forecast a decline in December mainline RASM of 2.5% to 3.0%.

Continental shares ended Monday's session up 42 cents, or 3.1%, at $13.96.