The Opera Theatre of St. Louis will be the first American company ever to perform at the Edinburgh International Festival, when ''Fennimore and Gerda'' and Stephen Paulus's ''The Postman Always Rings Twice'' are staged at the King's Theatre in September.
Meanwhile, Opera Theatre's eighth season will open May 28 with a revival of the 1979 production of Verdi's ''La Traviata,'' the first of five productions in its 1983 spring season at the Loretto-Hilton Theatre here.
The St. Louis season continues with ''Don Giovanni''; Berlioz's last opera, ''Beatrice and Benedict''; and a double bill of Francis Poulenc's ''Les Memelles de Tiresia'' and the world stage premiere of Frederick Delius's ''Margot-la-Rouge.''
''La Traviata'' will be directed by Colin Graham, Opera Theatre's associate artistic director. The conductor will be Henry Lewis, making his first appearance in St. Louis.
Opera Theatre of St. Louis was founded in 1976 by Richard Gaddes, general director.