Intracellular bacteria belonging to the genus Wolbachia have been described in filarial nematodes and these microorganisms appear to have evolved an obligatory mutualistic association with their filarial hosts. In fact, antibiotic treatment leads to the clearance of bacteria from worms resulting in a block in embryogenesis and, eventually, death of adult filariae. Currently, the antifilarial action of antibiotic treatment is interpreted as a secondary consequence of the bacteriostatic activity against Wolbachia endosymbionts. Here, we demonstrate by transmission electron microscopy the degenerative events occurring during embryogenesis of Brugia pahangi after tetracycline treatment. After 56 days of treatment the cytoplasm of hypodermal cords was totally void of Wolbachia and numerous vacuoles, residual of cytolitic activity, were observed. In the ovary, the morphology of the oocytes was well conserved 33 days after treatment, but the texture of symbiotic bacteria appeared altered. After 56 days of treatment embryogenesis was dramatically affected and the terminal portion of the ovary appeared totally empty. The authors suggest that the symbiotic bacteria play a direct role in worm metabolism and a long-term bacteriostatic effect may block bacterial activity involved in the active control of cytolysis. As a consequence, the bacteriophorous vacuole is transformed into a digestive vacuole and the whole symbiotic population is disrupted.