The microfilariae of Brugia tupaiae develop to the third stage in the thoracic muscles of Aedes aegypti (Liverpool strain) in 8.5 to 9 days. Larvae molt twice in the mosquito in the course of their development. The first molt takes place on the 5th day, the second on the 7th or 8th days. Third-stage larvae have average length-width measurements of 1.05 mm by 21 ,u. Tree shrews which received 17 to 75 third-stage (infective) larvae by subcutaneous injection, developed patent infections in 62 to 80 days. The larval stages of Brugia tupaiae are compared with and differentiated from those of other species of Brugia and other filariae. Brugia tupaiae, a parasite of the lymphatic system of tree shrews (Tupaia glis), has been reported from Malaysia, an area in which two related species, B. malayi and B. pahangi, are enzootic. The adults and microfilariae of these three species have been differentiated on morphological grounds (Orihel, 1966), but no information is available regarding the development of B. tupaiae in the intermediate or