Howard Mitchell, a cellist who was music director of the National Symphony Orchestra for 20 years, died Wednesday in Florida of heart failure resulting from surgical complications. He was 77.

Mitchell joined the Washington orchestra as principal cellist in 1933 under the first music director, Hans Kindler. Mitchell made his conducting debut with the National Symphony in 1941, and was appointed music director in 1949.Before he retired in 1969 as music director emeritus and guest conductor, Mitchell led the orchestra on its first foreign tour in 1959, a tour that featured 12 weeks of performances in 19 countries in South and Central America. The orchestra also toured Europe under his baton in 1967.