Abstract: Observations were carried out on the changes in the susceptibility of Aedes aegyptt (refm) to Brugia pahangi after receiving haemolymph from both refractory (repRR) and susceptible (refm) Aedes aegypti that were previously inoculated with microfilariae plus TC199 (tissue culture 199) or TC199 respectively. The metanization rates of microfilariae in the recipient mosquitoes were determined by dissecting the mosquitoes on day 3, 4 and 5 after haemolymph transfer. In the recipients receiving haemolymph from the refractory donors inoculated with microfilariae plus TC199, from the refractory donors inoculated with TC199 only and from the susceptible donors inoculated with TC199 only, the melanization rates of microfilariae were estimated to be 31.2%, 31.1% and 21.2% respectively. It is suggested that the intensity of melanization of mosquitoes varies with their susceptibijity inherited from their parents. Therefore, the melanization rates of microfilariae in the mosquitoes can be taken as one of the indices of their susceptibility. In addition, most (64.3-73.6%) of the recovered melanized microfilariae were found in the abdomen of the mosquitoes while almost all the developing larvae, in the thorax, possibly due to the choice of preferred sites for development in the latter.