In an attempt to study the occurrence of concomitant immunity in filarial infections, jirds (Meriones unguiculatus) were experimentally infected with Acanthocheilonema viteae, and patent animals were superinfected with a defined dose of A. viteae stage 3 larvae (L3). Infected animals harbored significantly less worms deriving from the superinfection than the control group (P < 0.05, 56.2%, and 63.4% protection), as shown by analysis of female worms 6 wk after superinfection on the basis of their developmental status and their length. This protection was not due to contact with L3 antigens because a significant reduction of worm burdens deriving of a superinfection was also observed after subcutaneous implantation of a single female worm (P < 0.05, 40.2% and 64.9% protection). The induced protective responses target L3 and restrict their migration because an established infection resulted in a reduction of L3 recovery (95.6% and 94.3%, P < 0.001) from tissues of jirds at day 5 after superinfection. Other data show that L3 from a superinfection are trapped within eosinophil-rich granulomas, which is likely to create unfavorable conditions for the worms and to lead to later death. Taken together, established A. viteae-infections partially protect hosts against homologous superinfection by an immune-mediated mechanism and, thus, regulate the population density of the parasites within the host by concomitant immunity.