Mr. Hoyt, a Democrat, was first elected to the Assembly in 1974. He represented the 144th Assembly District, which includes part of Buffalo and the town of Grand Island.
A strong advocate of energy conservation efforts, he was best known as a devoted environmentalist who was the Legislature's leading opponent of New York's proposal to purchase hydropower from a Canadian utility that plans to create the energy by flooding a wilderness area in Quebec.
"At times like these, there are no words to convey the sense of loss, of pain," Assembly Speaker Saul Weprin said in a statement. "Bill Hoyt was a loyal friend and a dedicated legislator."
Richard M. Flynn, the chairman of the New York Power Authority, who had his share of quarrels with Mr. Hoyt, called him a "man of integrity who fought tirelessly for causes in which he believed."
Before his election to the Assembly, Mr. Hoyt served as a member of the Buffalo Common Council from 1970 to 1974. He ran unsuccessfully for Mayor of Buffalo in 1989, when he was defeated by James D. Griffin.
He was educated at Hamilton College and the Park School of Buffalo, where he taught history for 11 years before entering politics.