Abstract Bath treatments with mebendazolc (MBZ) are commonly used for control of pseudodactylogyrosis in intensive eel eulture systems in Europe. To assess the potential for selection of drug‐resistant gill parasites in these systems, the authors have conducted a number of sub‐therapeutic treatments of an anthelmintic naive population of Pseudodactylogyrus bini (Kikuchi, 1929) Gussev, 1965, and P. anguillae (Yin et Sproson, 1948) Gussev, 1965, with different eoncentrations of MBZ. Parasite populations subtherapeutically treated seven times during a 7‐month period with 0.05 and 0.005 mg MBZI⁻¹ developed a high and low relative resistance to MBZ, respectively. It was shown that low temperatures decreased the effect of MBZ. In addition, the benzimidazoles albendazole and triclabendazole produced a comparatively lower parasiticidal effect, and it is suggested that treatments at low temperatures or with low effect bcnzimidazoles will increase the risk of selection of drug‐resistant gill parasites in eel farms.