Maurice Kaplow, 90, the noted Philadelphia ballet conductor, died Tuesday, Aug. 11, of kidney failure at Pennsylvania Hospital, his family said.

Mr. Kaplow, of Center City, started violin studies with his father at age 3 and pursued a multifaceted musical career that included work as a composer and violist. But it was in the specialty-within-a-specialty job as ballet conductor that he made his most visible mark. He was with Pennsylvania Ballet from its start in the early 1960s until stepping down as music director in 1991.

He went on to establish a two-decade-long association with New York City Ballet, retiring in 2010 from a professional strong suit for which he was widely admired.

The job of ballet conductor requires some alchemy, a keen eye and ear that can take two potentially competing art forms and turn them into a single complementary interpretation.

“It has to be musically viable as well as choreographically viable, and it has to be good theater, and that all has to happen at the same time,” said Martha Koeneman, who was brought on as Pennsylvania Ballet’s pianist by Mr. Kaplow in 1973. “It’s a multilayered skill, but he had a particular gift for making it all touch the viewer’s heart and the listener’s heart. He really captured that in his performances, which is really lovely.”

“He was a true musician and true ballet conductor, of which there are very few and far between,” said former Pennsylvania Ballet principal dancer Melissa Podcasy.

Mr. Kaplow was born in Cleveland and earned an undergraduate degree from the Cleveland Institute of Music and a master’s from the Eastman School of Music. He played viola with the Louisville Symphony and the Rochester Philharmonic, and was a member of the Philadelphia Orchestra from 1956 to 1957, and then again from 1958 to 1963.

He studied conducting with Pierre Monteux at the Monteux School in Hancock, Maine, in 1963.

“Monteux was going to take my dad on tour, his ‘coming out’ party,” said Lawrence Kaplow. “Unfortunately, Monteux died and so there was no tour, and instead my dad formed his own orchestra, the Pennsylvania Orchestra, and [Pennsylvania Ballet founder] Barbara Weisberger heard about him and they eventually joined forces.”

The Pennsylvania Orchestra became the orchestra of the new Pennsylvania Ballet.

Among the highlights of his years with the company was Signatures, a series of vignettes with a score written by him to highlight specific dancers and groups of dancers; and Winter Dreams, an arrangement of lesser-known Tchaikovsky material woven into a score for Robert Weiss’ choreography.

In 1984, at the Academy of Music, Mr. Kaplow conducted the ensemble in a 20th-birthday celebration for Pennsylvania Ballet that featured Rudolf Nureyev, Suzanne Farrell, and Grover Washington Jr., as well a new ballet by Peter Martins.

“I loved working with Maury,” said longtime Pennsylvania Ballet violinist Karen Banos. In rehearsals, he didn’t concentrate on technically tricky passages, she said, since he figured it was the job of players to work out those passages alone in the practice room. Instead, he would focus on the sound of the orchestra.

“It wasn’t about being clean, it was about being beautiful,” said Banos. “He just wanted beautiful.”

“You have to be able to make music,” said Weiss, a former Pennsylvania Ballet artistic director. “You can’t just be a metronome because the tempos need to be a certain tempo. The music still has to come to life and be emotionally rich and all the things ballet music can be. Maury was really able to do that just about better than anybody.”

The conductor and Pennsylvania Ballet parted ways in 1991, but he had already begun a new relationship that would prove durable. In 1990, Mr. Kaplow was invited by Martins to guest-conduct for the New York City Ballet and he became a full-time conductor for the company in 1995. He was given the title of principal conductor in 2005, retiring in 2010.

In addition to his work with Pennsylvania Ballet, he conducted the Philadelphia Orchestra in Prokofiev and Rachmaninoff at the Mann in 1979 with dancers Galina Panova and Valery Panov. He also guest-conducted the London Philharmonic, Cleveland Orchestra, and Pittsburgh Symphony.

He was known for an easygoing nature, even in tough moments. “He left me in awe quite a number of times because he seemed to be unflappable,” said Koeneman. “I don’t know if I ever saw him get upset.”

At least part of his success in Philadelphia, players said, came from the atmosphere of camaraderie he created. “He would have gatherings at his house, and mingle with us in the pit,” said Banos.

Koeneman remembers one time in the early 1980s when Pennsylvania Ballet’s financial struggles led to late paychecks. The day the musicians were finally paid, Mr. Kaplow showed up with wine to celebrate with the musicians.

“He was a real gentleman and a real leader,” said Koeneman. “He made it a lot of fun.”

He and his wife, Judy, celebrated 60 years of marriage at home on Aug. 7. In addition to his wife and son Lawrence, he is survived by sons Dan and Brian, five grandchildren, and his sister, Linda Kaplow. Services were Thursday, Aug. 13, at West Laurel Hill Cemetery. Donations in his name may be made to Congregation Mikveh Israel,