In 1946 Della Casa sang the part of Zdenka in Richard Strauss's opera "Arabella" in Zürich, and again the following year when she sang for the first time at the Salzburg Festival where she was to sing many times during her career. After the performance, Richard Strauss himself said: "That girl will be "the" Arabella someday!" In the same year, she sang for the first time at the Vienna State Opera House, singing Gilda in Verdi's "Rigoletto". Soon she moved to Vienna and joined the ensemble of the Vienna State Opera House. In 1949, she sang at La Scala Opera House in Milan as Sophie in Richard Strauss's "Der Rosenkavalier" and Marcelline in Beethoven's "Fidelio". She was invited to move to Milan, but she chose to stay in Vienna.