The only condition he placed on the gifts was that he remain anonymous, so only a handful of people at the A.C.L.U. knew he was the donor. He did not respond to e-mail messages or a letter sent to him in care of his investment vehicle, the Quercus Trust.

Calls to the phone numbers listed on filings the trust has made with the Securities and Exchange Commission resulted in busy signals.

Mr. Romero told the organization’s national board about the loss of Mr. Gelbaum’s money at its last board meeting in October, breaking the news in executive session. Mr. Romero did not reveal Mr. Gelbaum’s name, describing him only as “a donor,” board members said

Still, it is hard to keep secrets with a board of more than 80 members, most of whom report to state affiliates. “As soon as he started telling us, anyone who had a laptop with them was busy Googling” and figured out who the donor was, a national board member said.

Mr. Romero told the board that the donor had also stopped giving to three “sister organizations,” a phrase board members said he had used in the past to describe other groups with which the A.C.L.U. has collaborated, like the Sierra Club.

Mr. Gelbaum took a rare turn in the spotlight earlier this decade when environmental activists said he was behind the Sierra Club’s decision to adopt a neutral stance on immigration. Some people believe immigration has aggravated environmental problems.

He had given the organization a total of $101.5 million, according to The Los Angeles Times, which wrote what is perhaps the only major profile of him, in 2004. In the article, he is quoted as saying that he told Carl Pope, the Sierra Club’s executive director, in 1994 or 1995 “that if they ever came out anti-immigration, they would never get a dollar from me.”