Philippe Cousteau, 37, the younger son of Jacques-Yves Cousteau, the oceanographer and producer of films about the sea, was killed Thursday when the seaplane he was piloting crashed on landing in the Tagus River here.

Police said seven passengers aboard the plane, a World War II vintage Catalina, were rescued by fishermen. They said the aircraft struck a sand bank and flipped over on its back. Efforts to remove Mr. Cousteau before the plane sank were unsuccessful. Rescuers said he may have been knocked unconscious when the plane turned over.

Mr. Cousteau was a photographer, author and diver. He was born at Toulon, France and made his first aqualung dive in the Mediterranean at the age of 4. But throughout his life he devoted as much time to flying as he did to underwater exploration. He earned a glider pilot's license at the age of 16.

In 1976, he narrowly escaped death in the crash of a helicopter on Easter Island. The Catalina that crashed Thursday was used by the Cousteaus to film bird migrations and river courses.

Mr. Cousteau had dived in every ocean in the world and shared the experience with millions of people in his television films, photographs and lectures.

He received a degree in science from the College de Normandie and then spent a year studying electrical engineering at the Massachusetts Institute of Technology. He returned to Paris to train as a cinematographer.

During all his school vacations he worked with his father as photographer and diver aboard the Calypso, his father's research vessel. He worked on the award-winning feature film "World Without Sun," and was one of six "oceanauts" who lived 235 feet below the sea for 28 days during the historic Conshelf III Project in 1965. The experiment was filmed for a National Geographic television special on CBS.

Mr. Cousteau began filming "The Undersea World of Jacques Cousteau" television series in 1968 for ABC. He was vice president of the Cousteau Society and overall director of the Cousteau TV series, which won 10 Emmy awards.

He also was coauthor with his father of the book, "Sharks," publised in 1969.

Survivors include his wife, Jan, and a daughter, Alexandra, of the home in Marina del Rey, Calif. CAPTION: Picture, PHILIPPE COUSTEAU