Abstract: A longitudinal surveillance was carried out in Shijing Township, Zhengan County and Longpan Village, Dejiang County, where bancroftian filariasis had been virtually eradicated and control measures no longer adopted.The results indicated that the microfilaria rate (MR) in Shijing Township was further reduced from 0.15% in 1980 to 0.07, 0.04 and 0.05% in 1981, 1983 and 1985, the average Mf density in positive persons was 6.5, 11.5 and 3.5/120mm3 respectively. Since 1981, no new infections were found. The positive rate of IFAT was reduced from 8.9% in 1980 to 3.1 and 1.2% respectively after 3 and 5 years. From 1982-1985, annual dissection of Culex pipiens fatigans failed to find filaria larval infection. These results suggest that filariasis transmission in this area has been interrupted (Table 1).Concurrently another follow-up observation on the filariasis control was conducted in Longpan Village where microfilarial rate has dropped from 0.8% in 1975 to 0.48% in 1985, but due to the presence of a case with high microfilaremia density (72/120 mm3), the natural infection rate of vector mosquitoes collected from this household was as high as 20.6%, bearing with infective larvae. Blood examination discovered one new patient. The positive rate of IF AT was 23.5%. These findings showed that the tran-smission of filariasis in this area is continuing.The authors deemed that when MR was reduced to below 0.5% in an area, control measures might be omitted while surveillance should be strengthened.