Abstract. The European eel, Anguilla anguilla L., is able to respond to infections with ectoparasitic gill monogeneans of the genus Pseudodactylogyrus Gussev, 1965. A primary infection offers the host a relative resistance to challenge infections. The relative protection was detected at both 14 and 33 days after mebendazole termination of the primary infection by a significantly greater parasite reduction in previously infected eels compared to non‐primed eels. No complete immunity was obtained.