L. William Seidman was sworn in as chairman of the Federal Deposit Insurance Corp. Monday, replacing William M. Isaac who for more than a year had been urging the Reagan administration to replace him.

Within hours of the ceremony, Seidman was on a plane to New Orleans for his debut before the American Bankers Association, the industry's biggest trade association, which is holding its annual convention there this week.

Seidman's agency, among other things, insures customer deposits up to $100,000 at most of the banks that are members of the association.

Seidman, 64, is no stranger to Washington. He is a longtime friend of former president Gerald R. Ford, who tapped Seidman to be special assistant to the president for economic affairs from 1974 to 1977. Before joining Ford's staff, he was managing partner of Seidman & Seidman, a large accounting firm.

The FDIC not only insures deposits in nearly all the nation's banks, it also regulates the 8,500 state-chartered banks that are not members of the Federal Reserve System.

Seidman assumed the FDIC post during the most tumultuous period in U.S. banking since the Great Depression. Hundreds of banks have failed in the past several years, 95 of them this year alone. On one difficult day last summer, a record seven banks failed across the country.

Isaac, a Republican, was named to the FDIC board by President Jimmy Carter in 1978 and became chairman in 1981. He will stay around for several weeks to give Seidman time to get his feet wet. Isaac's assistant, Margaret Egginton, will remain at the FDIC as Seidman's deputy.

The White House originally tried to replace Isaac in early 1984 when his six-year term expired, but was blocked by top Senate Republicans. They told Reagan administration officials that they trusted Isaac, who had said he wanted to remain chairman for another year. Last fall, the administration decided that it was willing to nominate Isaac for another term. But Isaac, who by then had served eight months beyond the technical expiration of his term, declined the nomination.

DISCLOSURE RULES . . . H. Joe Selby, who has been acting comptroller of the currency since May, said this week that his office will propose a regulation to require federally chartered banks to issue periodic reports on their financial health. The banks will be required to reveal to the public a five-year financial summary, management analysis of the bank's condition, the status of outstanding litigation, insider trading of the bank's stock and executive compensation, among other things. National banks that are owned by publicly traded bank holding companies already are required by the Securities and Exchange Commission to make similar disclosures. But about one-third of the banks regulated by the comptroller's office are not owned by holding companies and thus are not covered by the SEC rules.