Southwest Airlines didn't waste any time expanding in the Twin Cities.

Just 3 1/2 weeks after starting up flights to Chicago's Midway Airport, the Dallas-based carrier will announce today that it will offer three daily nonstop round-trip flights from the Humphrey terminal to Denver starting May 26. The one-way fare, not including taxes and fees, will be $89.

Ticket sales to and from Midway "have been ahead of our expectations," said Bob Jordan, Southwest's executive vice president of strategy and planning, who declined to give specific numbers. "Opening Denver gives us a great opportunity to connect customers to the West."

With its latest move, Southwest is taking aim at more than Eagan-based Northwest Airlines. United Airlines flies here from its hub in Denver, and low-fare competitor Frontier Airlines has two flights daily. All three offer nonstop, one-way fares priced close to Southwest's offering.

Jordan said Southwest aims to "be very competitive. We're going to be the low fare in the market."

Southwest has moved aggressively in new markets to grow from a start-up low-fare carrier to the No. 1 airline for domestic travelers. Northwest, which dominates the gates at Minneapolis-St. Paul International Airport and is merging with Delta Air Lines, has thwarted efforts to crack the local market for years.

Southwest began its Humphrey-to-Midway service March 8, offering eight daily flights each way. From Midway, passengers can connect to 57 other cities on 197 daily departures. From Denver, passengers can fly nonstop to 32 cities, mostly in the West and Southwest.

After going into Denver three years ago, Southwest quickly built up to the 115 daily flights it offers now. But Jordan said the airline is taking a wait-and-see approach in Minneapolis-St. Paul, based on the economy, fuel prices and the softening of the airline industry.

"When we went into Minneapolis, we talked about being very conservative and we set the expectation ... not to expect a Denver kind of growth, obviously. But here we are three weeks later and we're talking about adding three more flights [to Denver] on top of the eight [to Chicago]," he said. "I think if it makes sense, we're going to do it. If it makes sense to add, we will."

Frontier flies four times daily to Denver from the Twin Cities; Northwest offers three flights, United five. On a randomly selected date of June 1, Northwest and Frontier listed nonstop, one-way fares on their websites of $109.60; United was at $99.60. Southwest's fare, which will be available starting at 10 a.m. today on its website, will be $103.20. All fares include taxes and fees, and some restrictions apply.

Suzanne Ziegler • 612-673-1707