With Ryan Costello's win Tuesday in the Sixth District congressional race, Chester County is on the hunt for a new member of its Board of Commissioners.

On Thursday, Costello submitted his resignation to Chester County Court, effective Dec. 15.

Costello, who is in his first term as a commissioner, will represent parts of Chester, Montgomery, Berks, and Lebanon Counties when he replaces Rep. Jim Gerlach (R., Pa).

Costello, a Republican, is the commissioners chairman. He chose to make his resignation effective after the Dec. 9 meeting, at which the commissioners plan to approve the budget.

"I've enjoyed learning the nuts and bolts of county government," Costello said. "I think that will be of tremendous benefit in my new position.

"I'm looking to take the priorities and values of this area down to D.C., and not vice versa," said Costello, who was born and raised in Chester County.

Commissioner Kathi Cozzone, a Democrat, lost her state Senate race for the 44th District on Tuesday. She is in her second term as a county commissioner.

The county court soon will issue a public notice requesting resumés and will set a date for interviews, open to the public.

Under Pennsylvania law, the court will fill the vacancy with another Republican, who will finish Costello's term, which expires in early 2016. Voters will elect a commissioner for a full term next November.

The court appointed Costello to the board in February 2011 to finish the term of Carol Aichele, who resigned the month before. Costello was elected to a four-year term the following November.

As of Thursday afternoon, two people had told the Chester County Republican Committee that they were interested in the job: Michelle Kichline, former chair of Tredyffrin Township's board of supervisors, and county Treasurer Ann Duke.

"I think having a woman with executive experience would be a great pick," said Val DiGiorgio, chair of the county's Republican Committee. "They each bring a lot to the table. They're both strong candidates."

mbond@philly.com

610-313-8207 @MichaelleBond