This paper describes a new species of earthworm (Pheretirna parualexandri sp.n.) which was found in the Mekong Delta, Vietnam. In addition, this paper also discusses the taxonomical position of Pheretima polychaetifera Thai, 1984. Pheretima parualexandri Nguyen, sp.n. is the largest sized worm in the Mekong delta. Firstly, dorsal pore in 12/13 or 13/14. Spermathecal pores four pairs in 5/6 - 8/9. Genital papillae present, paired in XIX. Genital papillae four pairs in VI - IX or less than. Behind genital papillae there is a setal zone (only present in XIX or both XIX and XX). Septa 5/6 - 7/8 and 10/11 - 12/13 thickened, 8/9 and 9/10 absent. Intestinal caeca, simple. in XXVII. Pheretima polychaetifera Thai, 1984 is a synonym of Pheretima alexandri (Beddard, 1901).