Effect of different numbers of infecting Brugia malayi on the survival of Anopheles sinensis were quantitatively studied in our laboratory. Four groups of healthy adult female mosquitos were tested. They were named G-0, G-1, G-2 and G-3, in which the numbers of microfilariae (mf) infecting per mosquito were 0, 5, 10 and 50, respectively. The experimental infection was conducted by inoculating the mf into the bodies of mosquitos through the neck membrane with a microinjector. It was observed that, in the groups from G-0 to G-3, the maximal life-span postinoculation (PI) were 21, 21, 20 and 10 days, the average life-spans PI were 7.78, 7.98, 7.05 and 3.55 days, and the survival time at 50% mortality PI were 6.72, 6.80, 6.40 and 4.00 days, respectively. Daily survival rates in the groups G-0, G-1 and G-2 declined slowly, over 25% on the 10th day PI, whereas the one in the group G-3 dropped down quickly, to zero on the same day. Linear regression analysis on the daily survival rates against the days PI showed significant differences between the groups G-0 and G-3 (0.02 > p > 0.01), but no significant differences between the groups G-0 and G-1, or between G-0 and G-2 (p > 0.5).