The Harvard African Expedition of 1926-1927 was planned for the purpose of making a biological and medical survey of Liberia, which apparently was that country of Africa about which the least was known in those respects. It was also planned, after the survey was completed, to cross the continent of Africa from the west to the east coast, travelling particularly through the Belgian Congo, and to make comparative studies in these regions. This program was carried out. This publication is the result of this expedition.