Manhattan's longtime district attorney, Robert M. Morgenthau, said Friday he won't seek re-election later this year after 34 years in office.

At a press conference, Morgenthau, 89, said he plans to complete the remaining 10 months of his term and then retire.

Morgenthau said he was unsure of his future plans, but also said he wasn't retiring for health reasons and that he is in good physical shape.

"Some people are slow learners," Morgenthau said. "It just took me a long time to realize I was getting older."

Morgenthau has been a major force in New York's legal community for decades, serving as U.S. Attorney for the Southern District of New York and winning nine consecutive terms as Manhattan's district attorney.

"The challenge of his successor will be to fill the shoes of a giant," said Charles A. Stillman of Stillman, Friedman & Shechtman PC.

Stephen E. Kaufman, of Stephen E. Kaufman PC, said New York is "losing the best district attorney we ever had."

Morgenthau displayed his characteristic wry wit as he made his announcement, saying his 93-year-old brother recently emailed him: "it's a bad time for you to be looking for a job."

Flanked by his wife and key members of his staff, Morgenthau said he expects his office to be very active in the coming months.

"We've got a lot of good cases in the pipeline," Morgenthau said.

Morgenthau served in the U.S. Navy in World War II and began his legal career in 1948, after graduating from Yale Law School, joining Patterson, Belknap, Webb & Tyler LLP.

After 12 years of practicing corporate law, he was appointed U.S. Attorney for the Southern District of New York by President John F. Kennedy Jr. in 1961 and created its much-touted securities fraud unit.

"He's the father of white-collar prosecutions," said Gary P. Naftalis, of Kramer Levin Naftalis & Frankel LLP, who worked as an assistant U.S. attorney under Morgenthau. "Nobody really did it before him. He set the standard for all that."

Morgenthau resigned as U.S. Attorney in 1970 after initially refusing to step down after Richard Nixon was elected president in 1968. U.S. attorneys traditionally offer their resignation when a new administration comes into office.

"If Richard Nixon had asked me to stay, I would have quit immediately," Morgenthau said.

Morgenthau won his first election as district attorney in 1974, replacing Frank S. Hogan, who had been district attorney for 32 years.

As district attorney, Morgenthau greatly increased the reach and breadth of the office, including pursuing international prosecutions and giving it a focus on white-collar crime it didn't previously have.

John W. Moscow, who predated Morgenthau at the district attorney's office and worked with him for 30 years, called Morgenthau "the best prosecutor we have had around."

"He's been the embodiment of integrity, toughness, fairness and decency," said Moscow of Baker & Hostetler LLP. "His integrity is far too great for the current times. He does things because they're lawful, because they're true."

Morgenthau's office brought dozens of high-profile cases over the years, including ones involving rogue bank Bank of Credit & Commerce International; defunct brokerage firms A.R. Baron & Co. and Duke & Co.; and the former top executives of Tyco International Ltd. (TYC).

In December 1991, liquidators for BCCI agreed to forfeit all of the bank's assets - $550 million - as part of a settlement with the U.S. Justice Department and Morgenthau's office. Sheik Kamal Adham, a major BCCI shareholder, in July 1992 pleaded guilty to a federal bank fraud charge and agreed to pay $105 million in fines.

A year later, Saudi Arabian financier Sheik Khalid bin Mahfouz agreed to pay $225 million to settle charges he conspired to steal more than $300 million from BCCI's depositors under an agreement with the Federal Reserve Board and Morgenthau's office.

In December 1997, A.R. Baron & Co., the defunct firm, pleaded guilty to a fraud that bilked investors out of $75 million, including manipulating initial public offerings and after-market trading in stocks. Twelve persons pleaded guilty and one was convicted at trial of criminal charges.

In the Duke & Co. case, the defunct firm's chairman, Victor Wang, and more than a dozen former employees pleaded guilty to criminal charges in a scheme to manipulate IPOs.

More recently, former Tyco top executives L. Dennis Kozlowski and Mark H. Swartz were convicted in 2005 on charges they systematically looted the company, which grew out of a sales-tax fraud inquiry by Morgenthau's office. Kaufman represented Kozlowski and Stillman represented Swartz.

Last month, Lloyds TSB Group PLC's (LYG) banking unit entered into a deferred prosecution agreement with Morgenthau's office and federal authorities and forfeited $350 million in connection with an alleged scheme that allowed clients in Iran, Sudan and other countries to access the U.S. banking system, in violation of U.S. sanctions on those countries.

Morgenthau also influenced pop culture, serving as the model for the character of Adam Schiff, the original district attorney on NBC's long-running television series "Law & Order."

The secret to his longevity: "Luck," Morgenthau said.

The departure of Morgenthau will open up a position that rarely turns over: Only two men have served in the office since Thomas E. Dewey left.

Leslie Crocker Snyder, a former judge who has previously challenged Morgenthau; Cyrus Vance Jr., the son of Jimmy Carter's secretary of state and a former prosecutor; and Richard Aborn, another former prosecutor, have all announced their intention to run.

There's also speculation that Dan Castleman, one of Morgenthau's top lieutenants, may run. Castleman declined comment Friday.

"There are some able and experienced people who are contending for the position," said Mark C. Zauderer of Flemming Zulack Williamson Zauderer LLP.

Whoever runs, Morgenthau said he doesn't plan to be active in the campaign.

- By Chad Bray, Dow Jones Newswires; 212-227-2017; chad.bray@dowjones.com