Abstract: Annual transmission potential was estimated as an indication for the intensity of bancroftian filariasis in Queshan County, Henan Province. The results were as follows.The mosquito biting rate was related to the mode and degree of exposure to mosquito at night. The average number of mosquito bites per person per night in Aug. -Sept. for those sleeping outdoors and those sleeping in the mosquito net indoors were 44 and 11 respectively, the former being similar to that for those sleeping indoors without mosquito net.The transmission season of bancroftian filariasis was estimated to be about 110 days. The intensity of transmission was highest in July, the number of infective larvae inoculated per person was 57.The annual transmission potential of filariasis in the three brigades investigated showed that among villagers sleeping outdoors the highest number of infective larvae inoculated per person per year or per transmission season was 95, while the lowest was 12; for those sleeping indoors with mosquito net, the highest was 20 and the lowest, 3.