It takes no more than 15 unionized musicians to play enough rock ‘n’ roll for an Elvis Presley-inspired Broadway musical, a federal judge ruled yesterday.

The ruling, by Judge Kenneth Karas of U.S. District Court in Manhattan, sides against New York’s musicians’ union. The union, the Local 802, had argued that the 2005 show, “All Shook Up,” had to employ a minimum of 18 in the orchestra pit.

The case began with a question that is sure to arise again in New York theater circles: Is the traditional Broadway full-pit orchestra usually required by the musicians’ union contract likely to interfere with efforts to capture the early-Elvis sound?

Judge Karas declined to rule on that. Instead, in a 15-page decision filled with references to Elvis’s songs, Judge Karas focused on a 2003 agreement between the union and the League of American Theatres and Producers, Inc. The agreement sets the minimum number of musicians that producers must hire when putting on a Broadway musical. The numbers vary for different theaters. Disagreements over those numbers prompted a strike in 2003, shutting Broadway down for nearly a week.

The current case involved the first attempt since the strike in which a producer sought an exemption to those minimums on artistic grounds, the director of labor relations for the League of Theatres and Producers, Seth Popper, said.

A portion of the collective bargaining agreement is set to expire in March 2007. Judge Karas wrote, citing the title of one Elvis tune: “The Court can only say, ‘Look Out Broadway.'”

The musical,”All Shook Up,” is a 1950s love story, which closed in September 2005, following a six-month run. It played with only 15 musicians — one drummer, one percussionist, one bass player, a pair of guitarists and keyboardists each, and eight trumpeters, trombonists and reed musicians, Mr. Popper recalled.

At the Palace Theater, where the show played, the agreement calls for a minimum of 18 musicians. The producers invoked a clause in the agreement that allowed for exemptions to the minimum when artistic considerations were involved.

“We believe that “All Shook Up” was a special situation,” Mr. Popper said. “The musical team was trying to express the totality of Elvis’ s work, with a gospel overlay.”

In 2004, an arbitrator, Carol Wittenberg, decided those artistic concerns were enough to allow the show to go on with only 15 musicians.

The only question that remained when the show closed was whether the union would fight for payment on behalf of the three musicians who were never hired. Mr. Popper estimated that hiring three musicians would have cost about $400,000 over the course of the year.

“No longer under the bright lights of Broadway, the Elvis Presley inspired musical ‘All Shook Up,’ finds itself where many successful Broadway shows end up: in federal court,” Judge Karas narrated at the beginning of his decision. The case arrived before Judge Karas after the union filed suit in response to the decision by the arbitrator.

Judge Karas found that the arbitration was grounded in the 2003 agreement. Federal judges invoke a high standard before overturning arbitration awards and Judge Karas ruled that he must affirm the arbitrator’s finding.

Had Judge Karas ruled otherwise, Mr. Popper said, the union could seek payment equal to the salaries and benefits that three musicians would have received on the show.

The lawyer for Local 802 did not return repeated calls for comment.

In his 15-page decision, Judge Karas took care to entertain his audience. The decision is full of meticulously footnoted quotations of Elvis song titles. Among Judge Karas’s judicial findings: the union arbitration produced “A Mess of Blues” (RCA 1960); the Local 802 requested that he take the case, “Patch It Up” (RCA 1970), and then “Return to Sender” (RCA 1962).

Since 2003, producers of about halfa-dozen musicals have gone to the union seeking exemptions to hire fewer musicians, Mr. Popper said.