To describe the discovery of a residual foci of bancroftian filariasis in Fuchuan County where the disease was announced to have been eliminated, and reveal its epidemiologic feature.The investigation was carried out from August 2007 to March 2008 among residents in Changtang village where the first case of filariasis was found and the neighboring villages. They were screened with two thick blood smears. Immunochromatographic technology (ICT) was conducted for those going out but returned and those in surrounding areas. Vector mosquitoes were collected and dissected to find filaria larvae. Historical documents were reviewed and relevant people were interviewed.In Changtang administrative village, 1052 residents were screened and 19 cases with microfilaremia were found in 2 natural villages, with a Mf-positive rate of 1.8% (5.1% in Gangshang and 1.4% in Yinshan respectively). No Mf-positive case was found in 4119 residents screened in other 3 villages. The average microfilaria density in the 19 cases was 17.37/60 +/- 1 blood. All the 19 cases belonged to 12 families, and 13 cases were relatives to each other, which showed a feature of spatial clustering and family clustering. More patients were identified in the age groups of 20-29 and 50-59, and 57.9% of them were older than 50 years. No larvae were found in 54 Culex pipiens fatigans dissected.The Changtang village is identified as a residual focus of bancroftian filariasis with a low, limited endemicity. More cases have been among the elderly with a low average microfilaremia.