DENVER  A huge patchwork of privately owned forest in northwest Montana  much of it abutting wilderness, and together almost a third the size of Rhode Island  will be permanently protected from development under an agreement announced Monday by two private conservation groups, the Nature Conservancy and the Trust for Public Land.

The groups will pay $510 million for about 500 square miles of forest now owned by Plum Creek Timber, a lumber and real estate firm based in Seattle. It is one of the biggest sales of forest land for preservation purposes in United States history, conservation experts said.

About half the amount will come from private donations, the conservation buyers said, and about half from the federal government under a new tax-credit bond mechanism that was included in the giant farm bill recently passed by Congress over President Bush’s veto.

The bond mechanism was devised by Senator Max Baucus, Democrat of Montana. Mr. Baucus, his spokesman said, was approached about a year ago by representatives of the Nature Conservancy and the Trust for Public Land, who argued that development pressures were growing so intense that new tools had to be created to buy the Plum Creek properties if they were to be protected.