Leopold Stokowski and Dimitri Mitropoulos were the orchestra’s two conductors in 1949, with Mitropoulos becoming conductor in 1951. Mitropoulos often conducted new music and music that was not well known. In 1957, Mitropoulos and Leonard Bernstein served together as Principal Conductors until, in the course of the season, Bernstein was made conductor, becoming the first American-born-and-trained conductor of the Philharmonic.