Investigating the development of Eustrongylides ignotus in its definitive host would enable us to trace the complete life cycle of this nematode. Fourth-stage larvae isolated from naturally infected swamp eels (Monopterus albus) were used to infect domestic ducks (Anas platyrhynchos domestica [L.]). We observed that male and female worms exhibited different developmental patterns in host ducks. In males, the fourth molt occurred at day 1-2 post-infection (PI), after which they attained maturity on day 4 PI and died between day 7 and 9 PI. However, females underwent the fourth molt at day 2-4 PI, produced eggs from day 9 to 17 PI, and then degenerated and died. When compared to fourth-stage female larvae, adult females demonstrated a considerable increase in total body size with a 151% increase in average body width and a 17% increase in average body length. However, the increase in size of the male larvae was not as significant as that in females. The average body width in adult males exhibited only a 45% increase over that in the larval stage.