The distribution and phylogeny of Wolbachia in filarial species suggests that these endosymbiotic bacteria may be important in the biology of their filarial hosts. An experiment to falsify this hypothesis would be to treat filarial worms with antibiotics which are active against intracellular bacteria. Indeed, it has already been shown that tetracycline treatment inhibits development in a model filarial species (Brugia pahangi) at different stages of the life cycle, in both mosquito and mammalian hosts. Here we discuss these previous data and present new results on the effect of tetracycline on the embryogenesis of the canine filaria Dirofilaria immitis.