-SINCE DISBANDING HER COMPANY last fall, dancer-choreographer Twyla Tharp has been laboring for the American Ballet Theatre, whose staff she joined at the invitation of Baryshnikov. Her haul so far has included creating three new pieces and restaging two of her best from yesteryear. In December she took a couple of weeks off to mount two works for the Paris Opera Ballet, and she is finishing her autobiography, to be published by Christmas.

Tharp has stopped dancing, saying, "I'm just not performing. Nor will I be. It's another of those businesses where the returns are diminishing, and you have to work more for less."-OPERA AT THE BROOKLYN ACADEMY of Music has been going on for 1861, 23 years before the Metropolitan Opera, but received a shot in the arm with its presentation in January of the Welsh National Opera's "Falstaff" by Verdi. Princess Diana of Wales, the opera's patroness, attended one performance.

BAM president and executive producer Harvey Lichtenstein plans more imports from abroad. IN 1991 BAM will collaborate with the Metropolitan Opera in Gluck's 1692 opera, "Orfeo and Euridice," and a timely new opera by John Adams.