Health insurer Aetna Inc. said Wednesday that chairman and CEO Ronald A. Williams will retire next spring, and that company president Mark T. Bertolini will take over starting next month.

The Hartford, Conn., company said Bertolini, 54, will become CEO and be named to the board effective Nov. 29. The 60-year-old Williams will become executive chairman and will retire in April.

Aetna’s board plans to elect Bertolini chairman after Williams retires.

Aetna is the third-largest publicly traded managed care company based on membership and revenue, trailing UnitedHealth Group Inc. and WellPoint Inc. Health insurance is its main product, but the insurer also sells dental, group life and disability coverage.

Aetna employs close to 4,000 in the Philadelphia region, most of them in Blue Bell.

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