NEW YORK —  The on-again, off-again talks to merge two ultra-fast nonprofit Internet networks have ended again — for good this time, it appears.

Internet2 and National LambdaRail serve many of the nation's universities and research institutions by offering fast Internet connections that physicists, astronomers and other researchers need to exchange large amounts of data.

The two next-generation networks began with separate missions, but their technologies and services converged over the years. And Jeff Lehman, chairman of Internet2's board, said their clients backed the merger because the organizations largely served the same community.

Talks resumed this year, and a committee with top leaders from each network worked out a compromise in August.

Internet2's board approved it, despite misgivings by some of its board members, but LambdaRail's board sought more concessions. And last week, both sides called it quits.

"A lot of us are disappointed," said Internet2's Lehman, a member of the merger committee. "We knew it was a challenge, and we were hopeful that we would find a way to get to the end this time."

Many LambdaRail participants felt they had invested in the system and wanted greater returns, Lehman said. He said Internet2 offered as much financial benefits as it could.

LambdaRail Chairman Erv Blythe, in a statement, described his organization as an atypical nonprofit, one that tracked how much individual members gave and received. He said LambdaRail needed additional concessions to satisfy its obligations to contributing members.

"Under the circumstances, we agree that our respective organizations have no choice but to move forward independently," Lehman and Blythe said in a joint statement.