Two months after celebrating its 10,000th passenger with rosy predictions about the future, Allegiant Air, the only commercial service out of Worcester Regional Airport, has told the city it will cease operations here Sept. 3.

Maurice J. Gallagher Jr., president and chief executive officer of the Las Vegas-based economy airline, told City Manager Michael V. O'Brien by phone Monday that escalating fuel costs forced Allegiant to end its nine month stint ferrying passengers between Worcester and Orlando-Sanford International Airport despite 85 to 90 percent capacity.

"We're down but not daunted," Mr. O'Brien said last night of the latest in a long series of airlines to depart Worcester over the last two decades. Allegiant becomes the 13th commercial carrier to come and go since 1988, when five commercial airlines served the airport.

The airline has been making twice-weekly flights to Florida, charging from $79 to $179 for one-way tickets. Mr. O'Brien said Mr. Gallagher assured him passengers holding tickets for flights after Sept. 3 will be reimbursed or offered tickets on other airlines.

Mr. O'Brien said he was told by Mr. Gallagher that the plane used by Allegiant on the Florida flights, the McDonnell Douglas MD 80, is not fuel efficient at that distance, cutting into its profit margin per flight.

City officials wondered, however, if other factors might be involved, including Allegiant's decision to become a publicly traded company, and the airline's recent announcement of expansion of service this fall in eight cities in the Mid-West and West.