Abstract: 　Objective] To explore the persistence period of medium and higher density of microfilaremia and its role in the transmission of filariasis in areas with filariasis basically eradicated. [Methods] The residual microfilaremia was followed up and the population were regularly examined by thick blood film assay. Culex quinquefaciatus were dissected to determine the natural infection rate and density of filarial larvae. The filarial antibody levels were detected by using IFAT. [Results] Two cases with medium and higher density of microfilaremia were found to be microfilaremia positive for twelve years. The natural infection rate and density of filarial larvae in Culex quinquefasciatus dropped down year by year, one case of new microfilaremia was found in the sixteenth year. [Conclusion] The persistence period of residual medium and higher density of microfilaremia lasted for more than twelve years.The individual residual medium and higher density of microfilaremia cases still possess the potential for transmission of filariasis.

Key words: Bancroftian filariasis, microfilaremia, Culex quinquefasciatus, filarial larvae, transmission, infection.