NEW YORK -- Philip R. Lochner Jr., a commissioner at the Securities and Exchange Commission, said yesterday that he would be leavingthe agency next month rather than seek reappointment.

The announcement took many officials at the SEC by surprise, particularly since Mr. Lochner appeared to have enjoyed his work at the commission. He has served only 18 months.

Mr. Lochner was appointed last year to complete the five-year term of the commission's former chairman, David S. Ruder. Earlier this year, hehad told some colleagues that he hoped to be renominated.

"I'm leaving the commission because I have two kids who are five or six years away from college," Mr. Lochner said.