WASHINGTON (AP) _ A man who has made a career of finding important fossils of early man in Africa was honored Tuesday by the National Geographic Society with a gold medal presented by President Reagan.

Kenyan fossil finder Kamoya Kimeu, a close collaborator with the famed Leakey family of anthropologists, received the society's distinguished La Gorce Medal from Reagan at a White House ceremony.

In later ceremonies at the society's headquarters, president Gilbert M. Grosvenor called Kimeu ''perhaps the world's greatest fossil finder'' and praised the man's knack for finding tiny pieces of ancient bone in unlikely places.

The award, formerly known as the society's Special Gold Medal, in the past has been given to such explorers and scientists as Richard E. Byrd, Robert E. Peary, Amelia Earhart and the crew of the Double Eagle II, the first manned transatlantic balloon flight.

Richard Leakey, director of the National Museums of Kenya, who has worked with Kimeu for 25 years, credited the Kenyan with many of the important findings of his expeditions.

''Without Kamoya's assistance, none of my projects could have been so successful,'' Leakey said.

Kimeu started as a field worker for Richard's parents, Drs. Louis and Mary Leakey, in 1960 when he was 21 years old. Later, the son of a goat herder and the son of the famed anthropologists became partners in digs around west Africa, and made numerous important discoveries.

The latest of Kimeu's important findings was the nearly complete skeleton of Homo erectus discovered last year near Kenya's Lake Turkana. The 1.6 million-year-old skeleton of an adolescent boy is the best example of this human ancestor ever found.

Leakey recalled how Kimeu, who supervises operations on his digs, took a walk on a day off and found a tiny piece of skull in an ''unpromising'' area near camp that led to discovering the rest of the skeleton.

''Kamoya has a gift,'' Leakey said. ''He knows how to look and he is persistent. He desired some recognition.''

Kimeu, speaking in Swahili translated by Leakey, said he would continue to look for fossils of human ancestors because ''there is no other way people can know the truth about the past.''