J. Barry Griswell will step down as CEO of Principal Financial Group Inc. on May 1, the Principal board of directors announced this afternoon. Larry Zimpleman will succeed Griswell as CEO and retain the title of president. Griswell will continue as chairman.

Griswell, who turns 59 next month, joined Principal in 1988 as an officer and agency vice president. He was named president in 1998, became CEO in 2000, and was elected chairman of the board in 2002.

Zimpleman joined Principal in 1971 as a part-time actuarial student and became a full-time actuary in 1973. He was named president and chief operating officer in 2006. A native of Williamsburg, Iowa, Zimpleman received a B.S. in business administration from Drake University in 1973 and his M.B.A. from Drake in 1977.

Griswell noted that the move matches the timetable announced when Zimpleman was named president in May 2006. “I said then that it’s normally a two- or three-year process, and it’s been two years,” he noted. Griswell indicated that his stint as chairman might end in another year or so. “This is clearly a transition move,” he said.

“I think the demands on a person as CEO of a large publicly traded company are pretty significant,” Griswell said. “This is seven or eight years I’ve served as CEO, and I think it’s healthy after that amount of time to step aside and let someone else take the leadership role.”

When he started the job, he said, “I don’t think I appreciated the wear and tear it has on a person.”

Griswell said that when he eventually retires from Principal, he envisions writing a book and remaining deeply involved in nonprofit causes. “The nonprofit world is my No. 1 passion,” he said.

On a more personal level, “I have two wonderful granddaughters who are the love of my life, and spending more time with them would be great,” he said.

“I’m not one to be golfing or fishing every day,” Griswell said. “As long as I’m employed by the company, my time will be devoted to the interests of the company. When I actually do retire — I have not made plans for that yet. I feel a little bit like a college senior.”