Susan Donna Lessenco, 26, a member of the Save the Children Foundation who was assigned to Makthar, Tunisia, was found dead in her home there on Friday.

According to a communication from the State Department, the cause of death was asphyxiation resulting from the operation of a hot water heater. The State Department said it was not clear whether Miss Lessenco had died on Thursday or Friday.

Miss Lessenco, who was born in Boston, grew up in Takoma Park, Kensington and Bethesda. She graduated from Winston Churchill High School in Potomac in 1972 and from the University of Michigan in 1977. At Michigan, she majored in cultural anthropology and was a member of Phi Beta Kappa.

She was a research assistant for a book on urban blight in Michigan and later worked for ACTION in Washington. In 1979, she went into a master's program at the School for International Training in Brattleboro, Vt. The school, which is run by the Experiment for International Living, requires that students do field work.Miss Lessenco chose to do hers with the Save the Children Foundation and was assigned to Tunisia. She arrived there last April.

Miss Lessenco had been a volunteer in the 1972 presidential campaign of Sen. George McGovern of South Dakota, the Democratic nominee that year, and in other political campaigns. She was interested in ecology and had planned a career in the developing nations.

Survivors include her parents, Gilbert and Elaine Lessenco of Bethesda; a sister, Amy Bernard of Gaithersburg, a brother, Robert, of Bethesda, and a grandmother, Cecilia Yospy, and a great-grandmother, Ida Paul, both of Baltimore.

The family suggests that expressions of sympathy be in the form of contributions to the Save the Children Foundation, Westport, Conn., 06880. c