The head of a large health insurer has set up a vehicle for disposing of 9% of his stake in the company.

Ronald Williams, chairman of Aetna Inc., Hartford, has adopted a “prearranged trading plan.”

Rule 10b5-1, a section of the Securities and Exchange Act of 1934, gives officers and directors of public companies the ability to set up plans for selling specified amounts of stock.

“The plans may be entered into only when the officer or director is not in possession of material, nonpublic information and may be used to gradually diversify investment portfolios over a period of time,” Aetna says.

Williams to start exercising employee stock options this quarter, and he will start by selling 800,000 shares as part of his personal financial planning, Aetna says.

William now owns the equivalent of about 8.9 million shares of Aetna stock, according to National Under calculations.