The airfare war is already under way, but the dogfight officially begins Friday when go! airlines launches service, becoming the third interisland jet carrier in Hawai'i.

The low-cost carrier is a venture of Mesa Air Group Inc., the largest company to enter the exclusive interisland market dominated for decades by Hawaiian and Aloha airlines.

"There's the word 'monopoly.' There's the word 'duopoly.' But there's no word 'triopoly,' " said Jonathan Ornstein, Mesa's president and chief executive. "With a third carrier in the market now, it's sort of forcing this level of competition, which is clearly good for the consumer."

Phoenix-based Mesa is one of the nation's largest regional carriers, with 182 jets, 5,000 employees and annual revenues exceeding $1 billion. It flies to more than 170 cities under the America West Express, Delta Connection, United Express and other brand names.

Ornstein called his company "the real deal" and promises a long-term commitment to Hawai'i.

That's not good news for Hawaiian and Aloha, both recently emerging from Chapter 11 bankruptcy protection and under pressure to perform for new investors.

Both carriers have publicly brushed off their new competitor, noting that several interisland startups — Mid-Pacific, Discovery and Mahalo airlines — eventually failed, and others never got off the ground. FlyHawaii Airlines had planned to launch this year but pulled out with Mesa entering the market.

It's no secret, however, that Aloha and Hawaiian have paid close attention to Mesa, quickly matching the low-cost carrier's every promotion and discount, despite soaring fuel costs.

Mesa introduced $39 promotional one-way rates in March. Aloha and Hawaiian quickly followed suit. That's nearly half of usual airfares in Hawai'i. Mesa also recently announced a special $59 round-trip weekday rate, and that was also matched.

Industry analyst Bob Mann of New York-based R.W. Mann & Co. said Aloha and Hawaiian are sending a clear message: "You're not going to underprice us, and we will be competitive."

Thom Nulty, Aloha's senior vice president, said some of the promotional prices are irrational. "With fuel prices the way they are, some of these lower fares frankly just don't make sense. But we want to be competitive," he said.

Mark Dunkerley, president and chief executive of Hawaiian Airlines, said in a statement, "We intend to maintain our leadership in the Hawai'i market, regardless of who the competition turns out to be."

Go! must also compete with two small interisland carriers — Island Air and Pacific Wings — that operate turboprop planes and serve Hawai'i's smaller airports. With Hawaii Superferry Inc. planning to connect the islands starting next year, using two high-speed catamarans, there are more ways to island-hop than ever before.