Simon Jones, Nancy Opel, Charlotte Moore and Stephen Mohanan star, Sept. 11-13, in the York Theatre Company's concert version of Darling of the Day, the first production of this season's "Musicals in Mufti" series. Like City Center's "Encores!" series, "Mufti" casts an eye on neglected musicals through concerts stagings. The 1968 Jule Styne-Yip Harburg-Nunnally Johnson Darling of the Day is set in England in first decade of the century, where a celebrated painter escapes his success by assuming the identity of his deceased butler. Michael Montel directs.

Jones recently appeared in the Roundabout Theatre's You Never Can Tell. His other plays include The Herbal Bed, Private Lives and The Real Thing. Opel's credits include Triumph of Love and Mere Mortals. Kelly (Sept. 18-20), which comes next in the Mufti series, is from the same time period as Darling, 1967, and is also set at the start of the century. The Eddie Lawrence-Moose Charlap tuner is about a brash teenager -- modeled after 19th-century New York legend Steve Brodie -- who decides to jump off the Brooklyn Bridge on a bet. Donna Kaz directs. Brian d'Arcy James stars as Kelly.

Billion Dollar Baby, of 1945, was Betty Comden and Adolph Green's first project after On the Town. Morton Gould wrote the music. The musical follows the rise of a 1920s golddigger who climbs to the top only to get a shock at the end of the show. B.T. McNicholl directs.

All the concerts are performed at York's home in The Theatre at Saint Peter's Church in the Citicorp Center (Lexington Ave. and 54th St.). Audience discussions follow matinee performances. For information, call (212) 935-5820.

-- By Robert Simonson