William Donaldson will be retiring from his post as chairman of the U.S. Securities and Exchange Commission June 30.[@@]

Donaldson, who turns 74 Thursday, said he will be returning to the private sector, but, at press time, he had not given any details about his next job.

Donaldson took over as head of the SEC in 2003, at a time when the collapse of Enron Corp., Houston, had shaken the faith of investors in securities regulation.

“Thanks to the dedicated efforts of the many professionals who serve at the SEC, this period has represented an extraordinarily active and effective time for the agency,” Donaldson says in a statement about his decision to leave the SEC. “It may well be remembered as the most consequential and productive period in the commission’s history since its founding in 1934.

Before Donaldson became chairman of the SEC, he was chairman of Aetna Inc., Hartford.

Donaldson helped Aetna cope with investor anxiety about class-action suits against managed care companies. At one point shortly before Donaldson took charge, Wall Street appeared to be assigning a negative value to Aetna’s managed care operations. Investors now are giving Aetna, which now consists mainly of the managed care operations and some other group benefits operations, a value of about $23 billion.