Ultrastructural observations on the structure and distribution of endosymbi
otic bacteria within the tissues of Wuchereria bancrofti are described. In
female worms the organisms were observed in the lateral cords of the hypode
rmis, oocytes, developing eggs and in intrauterine microfilariae. Organisms
were also detected in blood microfilariae and in the intestine of third-st
age larvae. Bacteria were not observed in male worms.