American zoologist whose collection and classification of many new species of marine protozoans helped establish marine biology on a systematic basis.

Kofoid graduated from Harvard University (1894) and in 1900 began a long affiliation with the University of California at Berkeley. He became a full professor there in 1910, and for most of the time until he retired in 1936 he was head of the department of zoology. He was a central figure in establishing the Marine Biological Station at San Diego (now the Scripps Institution of Oceanography at La Jolla), serving as assistant director from 1907 to 1923. He is best known for his work on the dinoflagellates and tintinnids, important groups of planktonic protozoans. To collect these organisms, he invented two pieces of equipment that bear his name, the Kofoid horizontal net and the Kofoid self-closing bucket.