Robert Franklin Gates, 76, an area artist for nearly 50 years who taught art at American University for 30 years and chaired its art department in the 1950s, died of arteriosclerosis March 11 at Oak Meadows nursing home in Alexandria. He lived in Arlington.

Over the years, his watercolors had been exhibited in one-man shows at the Phillips, Corcoran, Franz Bader and Jefferson Place galleries in Washington, as well as at Howard and George Washington universities. His works also were displayed at the National Gallery of Art, the Art Institute of Chicago and the Metropolitan Museum of Art in New York City.

Institutions from which he received awards for his paintings included the Society of Washington Artists, the Corcoran Gallery of Art and the Baltimore Museum of Art.

An art critic writing in The Post in 1967 said that Mr. Gates was "a highly accomplished artist who draws beautifully, who understands color, who handles a wide variety of media with admirable skill. Mr. Gates is a professional. He paints loosely and freely, but his experimentation is always controlled. He improvises on traditional themes, single figures, landscapes and crowds."

Mr. Gates was a native of Detroit where he attended the Detroit School of Arts and Crafts before moving to New York and studying anatomy at the Art Students League. He also attended the Phillips Gallery Art School after moving to Washington in the early 1930s.

During the 1930s, he taught at Studio House in Washington, and worked for the Treasury Department's Section of Fine Arts. While working for Treasury, he painted murals for post offices in Bethesda and Oakland, Md., and in Virginia near White Sulphur Springs. He also traveled to the Virgin Islands and painted a series of watercolors commissioned by Treasury.

He served in a civilian capacity with the Office of Naval Intelligence during World War II, working on projects involving cartography, camouflage, and photography. He was awarded the Navy's Distinguished Civilian Service Medal for his wartime work.

Mr. Gates joined the faculty of American University in 1946, and served as chairman of its art department from 1953 to 1957. He was named University Professor Emeritus when he retired in 1975. During his years at the university he taught courses in painting, life drawing and grpahics.

From 1966 to 1967, he was artist-in-residence at the U.S. Embassy in Baghdad, Iraq.

His marriage to the former Margaret Casey ended in divorce.

Mr. Gates' survivors include his wife, Sarita Weekes Gates of Washington, and his stepfather Karl Miller of Vero Beach, Fla.