Referring to the sociological structure of the family in the Bateke ethnic group which most of the inhabitants of Kwamouth (Bandundu region, Zaïre) are descendants, the author defines the concept to family contamination in trypanosomiasis induced by Trypanosoma brucei gambiense. The calculated incidence in the selected population according to the different types of family and vicinity contamination was found to be at least five times higher than the incidence obtained for the global population. To explain this considerable difference, the author suggests the mechanical transmission of the trypanosoma, but feels that cyclic transmission plays a significant role, as the life in close community is characteristic for the inhabitants of this area.