Paul Arthur Zahl, 75, an explorer, author and photographer who as the senior natural scientist for the National Geographic Society discovered the world's tallest tree in California, photographed the world's first known albino gorilla in Africa, and found a flower as big as a washtub in the rain forests of Malaysia, died of cancer Oct. 16 at Greenwich Hospital in Greenwich, Conn.

Dr. Zahl was the Geographic Society's senior natural scientist from 1958 until he retired in 1975, and he had continued to serve on its committee for research and exploration until his death. He lived in Washington until 1979, when he moved to Cos Cob, Conn.

He was the author or photographer of more than 50 articles for the society's magazine, and before joining National Geographic, his explorations had included searching for giant ants in the jungles of Brazil, Venezuela and British Guiana in the late 1930s and research on the disappearing Bahamas flamingo in the early 1950s.

Dr. Zahl also explored the swampy waterways of tropical Venezuela in a dugout canoe looking for the breeding place of the little-known scarlet ibis, and did deep sea diving in the Straits of Messina between Sicily and Italy while researching deep-sea fauna.

He was "a distinguished marine biologist and a longtime valuable member of our research committee," said Dr. Melvin M. Payne, chairman of the board of the society and of its committee for research and exploration.

Dr. Zahl was born in Bensenville, Ill., and graduated from Illinois' North Central College. He earned a master's degree and a doctorate in experimental biology from Harvard.

In 1937 he joined a Harvard colleague, Dr. Caryl Haskins, in establishing the Haskins Laboratories in New York, a research organization that concentrated on biochemistry, biophysics and medical physiology. He remained at Haskins as staff physiologist and later as associate director until 1958 when he moved to Washington and joined National Geographic, but from the start his primary interest was travel and exploration.

His first book, "To the Lost World," the story of his search for giant ants in the South American jungles, appeared in 1939. Dr. Zahl brought 2,000 of the ants -- some as long as two inches -- to the United States to study in his laboratory. On a return trip to South America for National Geographic in the 1950s, he discovered some of the world's largest beetles -- some longer than six inches with pincers that could snap a pencil in two or cut a finger to the bone.

During World War II, Dr. Zahl served with the Office of Science Research and Development, then resumed his explorations after the war. He wrote about his search for the Bahamas flamingo in "Flamingo Hunt" in 1952, and of his search for the breeding place of the scarlet ibis in "Coro-Coro, the World of the Scarlet Ibis," in 1954.

In 1963 he found "the Mount Everest of all living things," a California redwood tree, which at 367.8 feet was considered the tallest in the world. He photographed the world's first known albino gorilla during an expedition to equatorial Africa in the mid-1960s where he also photographed frogs that weighed more than seven pounds and measured three feet from nose to toe.

"The forelegs were as thick as my wrist, and the powerfully muscled rear legs were larger than turkey drumsticks," Dr. Zahl wrote in a 1967 issue of the National Geographic magazine.

On a trip to the jungles of Malaysia, Dr. Zahl found a Rafflesia flower the size of a washtub with leather-like petals that measured 26 inches from edge to edge.

Between expeditions, Dr. Zahl had also done research for the National Cancer Institute, the National Science Foundation and the old Atomic Energy Commission. He was a member of the Cosmos Club here and the Explorers Club in New York.

Survivors include his wife, Eda, of Cos Cob; a son, the Rev. Paul F.M. Zahl of Scarborough, N.Y.; a daughter, Eda Kristin, of Los Angeles; two sisters, Frances Munson of Battle Creek, Mich., and Lillian Meloney of Escondido, Calif., and three grandsons.