1. Nine species of mosquitoes which commonly attack the human in the endemic area of filariasis in Amami Island, southern Japan, were experimentally tested for the susceptibility against bancroftian filarial worm. The 3rd stage or infective larvae were examined in four species of mosquitoes; Culex pipiens fatigans, C. tritaeniorhynchus, C. bitaeniorhynchus and Anopheles sinensis. Aedes albopictus, Aed. riversi, Aed. vexans and Armigeres subalbatus were non-susceptible and Aed. Japonicus failed to feed on microfilarial carriers. 2. The majority of ingested microfilariae failed to develop in the early phase, even in C. pipiens fatigans which was shown to be the most efficient host, a large number of microfilariae were lost during the first 36 hours after intake. However, during the same period, only a small part around 4.2% of microfilariae could be found in dejectas. This fact suggests that the loss of ingested microfilariae were mainly due to some processes other than dejection out of the mosquitoes. 3. The rate of infected mosquitoes and the rate of infective mosquitoes increased linearly in accordance with microfilarial density of the donor's blood. The regression line of the former is Y=41.654X+48.735 and that of the latter is Y=25.178X+48.735, where Y is the rate and X is the logarithm of microfilarial density. 4. Host efficiency of C. pipiens fatigans was tested at various densities of microfilaraemiae. Host efficiency of An. sinensis at the density of 1.21 Mf/cmm was 0.076 and that of C. tritaeniorhynchus atthe density of 1.32Mf/cmm was 0.010, whereas, those of C. pipiens fatigans at both densities were 0.75 and 0.67 respectively. This fact shows that C. pipiens fatigans is much more efficient as the host of bancroftian filarial worms than the other two species. 5. Mortality of C. pipiens fatigans increased with the increase of the number of larvae ingested, and it was particularly remarkable when the mosquitoes fed on the blood donor having the microfilarial density over 5.0Mf/cmm. For the most part of the mosquitoes, deaths occurred when the larvae reached to the 3rd stage and started to migrate through the body tissues of the mosquitoes. 6. The indices of the experimental infection were calculated after Kartman (1954) with C. pipiens fatigans at various numbers of intake of microfiariae. The highest ndex was obtained around two to three microfilarial intake per mosquito. Therfore, it was shown that the microfilarial carriers with the microfilarial densityof blood appropriate to serve the above mentioned number of microfilariae to mosquitoes are most dangerous as the reservoirs.