Abstract: A field investigation of critical transmission level of malayan filariasis was carried out in a low endemic area from 1982 to 1984 in Mojiaqiao village of Yuhang County, Zhejiang Province, where Anopheles hyrcanus sinensis acts as vector of the disease. Based on the findings of three consecutive years, we are convinced that a microfilarial density below 5/60μl peripheral blood is no longer significant in the spreading of the disease. And, if the microfilariemia rate is reduced to less than 1% and the microfilarial density is as low as 5/60μl or less, the remaining cases with microfilariemia would gradually be self-cured and the transmission of the disease would virtually be interrupted. Therefore, finding out and treating those subjects with high microfilarial density as soon as possible are key measures to prevent the spreading of filariasis malayi in low endemic area. In addition, there is no longer any need for repeated surveys and treatments. The necessary work is to strengthen monitoring activities. As a result, much manpower and material resources could be saved.