Joseph L. Sax, a legal scholar who helped shape environmental law in the United States and fuel the environmental movement by establishing the doctrine that natural resources are a public trust requiring protection, died on Sunday at his home in San Francisco. He was 78.

The cause was complications of a series of strokes, his daughter Katherine Dennett said.

In 1970, as Americans were becoming increasingly alarmed about pollution, Professor Sax emerged as one of the most prominent of a new breed of lawyers focusing exclusively on the environment.

That year, amid concerns about smog and contaminated waterways, 20 million Americans were mobilized to participate in Earth Day, and President Richard M. Nixon signed the Clean Air Act and legislation creating the Environmental Protection Agency, a year after the Council on Environmental Quality was created in the executive branch.

In his signal achievement, Professor Sax reached back to ancient Roman law to formulate a far-reaching legal doctrine that recognizes the air, seas and other natural resources as a public trust that must be protected from private encroachment.