In an area of central China where Brugia malayi is transmitted by Anopheles sinensis, three villages were followed for 4 years without any control measures. Diethylcarbamazine (DEC) had been used in a control programme reducing the parasite rate in Shuiwa from 12.72 to 0.59%, in Gubomen from 3.18 to 1.55% and in Moshi from 5.62 to 2.81% (although this fell the following year to 2.23%) up until the start of the trial. The village populations, the human biting rate, the parous and the gonotrophic cycle were comparable for all the areas, yet the microfilariae rate dropped to 0.18% in Shuiwa, 0.31% in Gubomen, but not in Moshi where it increased from 2.23 to 2.43%. This indicated that Shuiwa and Gubomen were below the threshold of transmission. Due to the small number of positive cases remaining, the density measurements showed less change, but a separate study on 44 individuals followed for 6 years revealed that those above 13 microfilariae per 60 mm3 remained unchanged or increased, while those below progressively decreased their densities. The threshold of transmission of B. malayi by An. sinensis was considered to be between 1.55 and 2.23% of the population, providing no individual had a higher density than 12 microfilariae per 60 mm3.