The Royal Ballet has the reputation for having the mose "traditional" version of "Swan Lake," but there is much that differs from what is fondly imagined to be authentic. Last night at the Kennedy Center, the company unveiled the Ashton fourth act, which interpolates music from the Tchaikovsky score unused elsewhere in the ballet instead of the usual extraneous additions.

Ashton gives the Prince and the Swan Queen a private pas de deux, interrupted by an enraged Von Rothbart who swoops upon Odette and tries to carry her off in his claws, like an owl with a mouse. When the two lovers have chosen death, depriving the Sorcerer of his evil powers, swans rush on from all directions to finish him off. The choreography is as lovely in its group dances as it is dramatic, the legato dancing of the swan maidens punctuated by sharp, jagged lifts for Odette and the Prince.

Taken as a whole, the company was in better form than on opening night. The soloists were as stylistically apt as they had been stylistically errant. Much of the credit goes to Anthony Dowell's impetuous, impassioned Siegfried. Dowell's partnering was superb and his solo dancing, both in Act I's difficult adagio and Act III's brilliant solo, was both fluid and crisp.

Marguerite Porter's porcelain prettiness and pelican limbs make her an ideal Odette and her majestic timidity worked well in the "white" acts. Fouette fans were disappointed by her substitution of pique tours in the third act pas de deux; dance fans were equally disappointed by the substitution's lack of brilliance.

The soloists in both classical and character roles were everything an opening night cast should be. Particularly notable were Rosalyn Whitten in the Act I pas de trois; Wendy Ellis in the Act III pas de quatre; and Judith Howe as one of the leading swans.