Lt. Col. (USAR ret.) and Mrs. Martin Gerard Tracy of Washington, D.C. announce the engagement of their daughter, Mary-Louise Agnes to Edward Osborne Coates III, Ens. USCG, son of Dr. and Mrs. E.O. Coates Jr. of Bowdoinham, Me. and Small Point, Me. The wedding is planned for October 20.

Miss Tracy, an alumna of Stone Ridge Country Day School of the Sacred Heart, is a cum laude 1976 graduate with a BS in biology from Georgetown University and an MA in biology from Boston University. She is an information specialist with the Systematic Entomology Laboratory, USDA, at the Smithsonian Museum of Natural History, and previously taught at Immaculata Preparatory School here. Her father, now retired, was an attorney for the Department of Defense.

Mr. Coates attended the Grosse Pointe (Mich.) University School and was graduated in 1970 from the Holderness School (N.H.). He attended both Williams College and the University of Arizona and received a degree in biology from Boston University. He is assigned to the Office of Marine Environmental Protection in Seattle, Wash., and has also served six years in the Naval Reserve.His father is assistant chief of staff at the Veterans Administration Hospital in Togus, Me.