The carrier began service to Louisville, Ky., and Green Bay, Wis., and is adding a third daily flight to New York-LaGuardia.
Tomorrow it will begin service to Branson, Mo.
Frontier also is beginning seasonal flights from Milwaukee to San Francisco and San Diego today. Milwaukee is the hub of Midwest Airlines.
Frontier and Midwest are now owned by Indianapolis-based Republic Airways Holdings Co.
It will take six to nine months to fully integrate the two airlines, officials said, and for now, passengers should check in with the airline that operates the plane they’re flying.
On May 14, Frontier plans to add a fifth daily flight from DIA to Portland, Ore.; San Francisco; and Seattle/Tacoma for the summer.