Abstract: Twenty one villages from ten provinces and one autonomous region with different mf rates in population and different levels of mean mf density per 60 μl blood in positive cases and representative species of filaria and vector after DEC treatment were selected as study sites.In these villages a longitudinal investigation for a period of 3-5 years was carried out without adopting any control measure.The main results were:1.Among 279 residual microfilaremia cases,235 (84.2%) turned negative spontaneously and successively.The negative conversion rates were 86.6% (46/53) in malayan cases,60.8%(28/46) in bancroftian cases after 3 years’ follow up, and 100% (29 /29) in malayan cases, 87.4% (132/151) in bancraftian cases after 4 5 years follow up. Among 110 bancroftian cases followed up to seven years,109(99.1%)turned negative.2.Twenty two new microfilaremia cases were detected,of which,18 might be missed in previous blood examination,4 might be new infection.3. Mf rate in the population in twenty villages has been trending down,whereas in one village fluctuated around the original level.Among the 20 villages,no microfilaremia case was found in 7 villages.4. The natural infection rates of filarial larvae and infective rates in mosquito vectors were in descending trend in most of these villages,and no filarial larva was found in 12 villages.It was demonstrated that under these circumstances where the man-biting rate of mosquito vectors was within 20-50 mosquito per person and night by using the method of human bait bed net trapping, and about 40% of the population used bed-net reasonably,the mf rates in the 20 study villages were found to be ≤1.71% in areas of bancroftian filariasis,≤1.55%in areas of malayan filariasis and the mean mf density of residual microfilaremia cases in most villages was 3-10 mf per 60 μl blood,moderate mf density was found only in individual case,filariasis transmission has already been brought down below the threshold,and tended to be interrupted.In one village of malayan filariasis,the mf rate in population was 2.31% and there was one case of microfilaremia with density of 54mf/60 μl,filariasis transmission was at the critical level.