The cause of death was Alzheimer's disease, according to his son-in-law, Paul Raymond of Howard, Pa.
Known nationally for his championing of American music, Mr. Hegyi came to Williams in 1965 from the Chattanooga (Tenn.) Symphony, where he served 10 years as conductor. He remained at Williams until his retirement in 1985.
Besides conducting the Williams-based Berkshire Symphony during those years, he served concurrently as music director of the Albany (N.Y.) Symphony Orchestra. At Williams, he founded and ran the Music in the Round chamber series and played in both it and the Williams Trio as a violinist. His wife at the time, Charlotte Hegyi, performed in both groups as pianist.
"He brought quite a few distinguished performers here" to perform with him in Music in the Round, according to retired music professor Irwin Shainman. Among them was pianist Garrick Ohlsson, a stalwart on the American music scene and at Tanglewood.
These were performers Mr. Hegyi knew personally, Shainman said.
"That was the old way of doing things," Shainman added. "You bring in your friends. You don't have to pay them money."
The performances were on a high level, often of new or difficult music such as Olivier Messiaen's World War II "Quartet for the End of Time," Shainman said.
It was Shainman who was largely responsible for bringing the conductor to Williams. Then conducting the Berkshire Symphony himself, he obtained a five-year Rockefeller Foundation grant to give the orchestra a professional leader on the podium.
Mr. Hegyi later founded a string instruction program at Mount Greylock Regional High School, Shainman recalled. He noted that the program continues to this day.
Mr. Hegyi in turn attracted cellist Douglas Moore to the college faculty to perform in the chamber music concerts. Moore went on to serve as principal cellist and often as soloist under Mr. Hegyi in both the Berkshire and Albany symphonies.
"When I got here as a cellist (in 1970), I found out I had to be a classroom teacher as well," recalled Moore, who is now retired. "There's quite a difference."
Moore also found himself in an "odd relationship" when, as chairman of the music department, he had to perform under his colleague's baton.
"He was a challenging Hungarian," said Moore, referring to Mr. Hegyi's parentage. "He always had the conductor's role in mind. I learned a lot about music from him also about conducting and rehearsal technique."
Mr. Hegyi was born Feb. 2, 1923, in New York. He performed his first concert on the violin at the age of 10 and graduated from the Juilliard School in New York. He played New York recitals in 1945 and 1946 and became violinist in the now-defunct Berkshire String Quartet.
A student of the noted Greek-born conductor Dmitri Mitropoulus, Mr. Hegyi played in several New York orchestras, including the Radio City Music Hall and New York City Ballet ensembles, before taking up a conducting career.
Among his conducting awards were first prize in the 1958 International Competition for Conductors in Liverpool, England, and the Alice M. Ditson award from Columbia University in 1983. The latter was for programming and promoting American music.
Among the many American composers whose work he premiered were Jacob Druckman, Charles Wuorinen and Tobias Picker. With the Albany Symphony, he also left a legacy of recordings of American music.
After his retirement from Williams, Mr. Hegyi went on to guest-conduct orchestras in the United States and abroad, including the New York Philharmonic (doubling as violin soloist), the Beijing Philharmonic, the Shanghai Symphony and orchestras in Romania, Puerto Rico and El Salvador.
Over the course of his career, tributes came from such musicians as Ohlsson, who performed repeatedly with him at the Albany Symphony as well as at Williams. After one Albany performance, Ohlsson said, "Julius was a master of all elements of music technical (as conductor or superb violinist), structural, emotional, spiritual."
Reviewing a Hegyi performance in The New York Times, John Rockwell wrote that, "given the Albany Symphony's venturesome commitment" to the cause of American music, the orchestra played Samuel Barber's Symphony "with a sure technical command that never got in the way of expressivity."
Mr. Hegyi leaves his wife, Nancy Hegyi; a daughter, Lisa Raymond; a sister, Louise Larson; and five grandchildren. A son, Lee David Silver, predeceased him.
Services will be private. Memorial contributions may be sent to the Albany Symphony Orchestra. Condolences may be mailed to Lisa Hegyi Raymond, 435 Ponderosa Drive, Howard, PA 16841.