Charles F. DeLong, 64, co-founder and chairman of the board of United States Trading International Inc., a Washington marketing firm, died Thursday at Georgetown University Hospital. He had a heart ailment.

Mr. DeLong was an FBI special agent during World War II. After the war, he worked as a creative marketing specialist in New York City for Economics Laboratory Inc., and for several cosmetic firms, including Rayette-Faberge, Revlon and Elizabeth Arden.

A Washington resident since 1974, he served as head of the export division for U.S. Consumer Products for the Commerce Department in 1975, the year he helped found United States Trading International Inc.

Mr. DeLong was born in Spring Valley, N.Y. He earned bachelor's degrees from St. John's University and Pace College, both in New York, and graduated from the American Institute of Finance and the FBI Academy. a

Survivors include his wife, Baroness Sophia Banszky DeLong of Washington; two daughters, Barbara Cameron of Mamaroneck, N.Y., and Virginia Elsenhens, of Philadelphia; two stepdaughters, Catherine Banszky of New York City and Anita Landa of Montpelier, Vt., and two grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the Georgetown University Heart Fund in the Division of Cardiology.