An arrest warrant was issued yesterday for the former assistant office manager and accountant of a Washington law office who is accused of defrauding the firm of at least $80,000 over a two-year period.

U.S. District Judge George L. Hart Jr. issued the warrant after Catherine A. Bethea, 29, of 3187 Apple Rd. NE, failed to show up for her arraignment on the fraud charges.

Assistant U.S. Attorney Brian W. Shaughnessy, chief of the fraud division, said the indictment against Bethea details an alleged fraud against the Washington office of the law firm of LeBoeuf, Lamb, Leiby and MacRae, at 1757 N St. NW. The firm also has offices in New York.

According to the mail fraud charges against Bethea, she had the responsibility at the law firm of preparing checks to pay bills, submitting the checks for approval to a partner in the firm, balancing the bank books, and transmitting the financial information to the firm's New York accounting office.

The indictment charged that between Dec. 6, 1974 and Dec. 9, 1976, she prepared checks in the names of real or fictional suppliers and forged the checks herself or told the firm they were legitimate checks. She would then alter vouchers in the office or send altered vouchers to New York to cover up the false entries, the indictment continued.

Once the paperwork was completed, she would cash the checks at a Washington bank where "she had established a banking relationship through her employment at LeBoeuf, Lamb," the indictment stated.