The effective formalin therapeutic concentration (ETC) against Cichlidogyrus spp. infecting tilapia was obtained. Also, the stress and immune response to formalin baths in O. niloticus were achieved. A static bioassay with four concentrations (0.5, 1.0, 1.5, 2.0 mL L-1) of formaldehyde (37% dilution) in water, and one control without this substance was performed. Results evidenced that 1.5 mL L-1 of formalin for 40 min was the ETC to eradicate monogeneans without manifesting macroscopic damage to fishes. Thirty tilapias underexposure with ETC of formalin (FORM Group) were evaluated for stress, utilizing cortisol and glucose determination, and immune response (lysozyme activity and differential counting of organic defense cells). Other 30 tilapias without cichlidogyriasis and with no exposure to formalin were the Control Group. Evaluation times were: three hour pre-treatment (T0), and one hour, three days and three months post-treatment (T1, T2, and T3, respectively). A peak of cortisol (~326 ng mL-1) was observed in tilapias of FORM Group at T1 with significant differences with the control group (P ˂ 0.05). Similarly, glucose concentration reached significant values (~119 mg dL-1) at one hour after the conclusion of treatment (P ˂ 0.05). Those values returned to the basal limit at the last time of the study. The lysozyme activity and white blood cells in differential counting showed no significant differences in the same conditions (P > 0.05). Results confirmed total control of cichlidogyriasis with the ETC of formalin obtained and an evident resistance of tilapia to treatment as stabilization of its physiological stress response was achieved.