Managed care company Aetna Inc. said Friday that Chairman Ronald A. Williams plans to retire on April 8, a move first announced in October.

President and CEO Mark T. Bertolini will succeed Williams. Bertolini, 54, already assumed the chief executive position previously held by Williams in November in a widely expected leadership change.

Williams, 61, served as company chairman and CEO since 2006.

The number of board members will be reduced to 13 from 14 after Williams' retirement.

Aetna is the third largest commercial health insurer based on enrollment, behind WellPoint and UnitedHealth.

Earlier this month, the company said its fourth-quarter net income rose 30 percent on better pricing and a slowdown in health care use that benefited the industry in the final months last year. The Hartford, Conn., company also predicted a 2011 operating profit that exceeded Wall Street's expectations.

Shares of the company fell 55 cents, or 1.5 percent, to $37.36 in afternoon trading.