With the objective of evaluating the effect of formalin and the aqueous extract of the leaves of Mansoa alliacea "ajos sacha" (EAHAS) in the control of monogeneous parasites present in adults of Otocinclus affinis, parasitological examinations were carried out on 400 specimens, analyzing skin, fins and gills; the monogeneans were fixed in permanent sheets using the Gray & Wess method, and their parasitic indices were determined: prevalence, average intensity, average abundance. For the bioassay of the lethal median concentration (LC50-96h), 720 specimens were used, distributed in a completely random design, in 6 treatments with 3 repetitions, evaluating the formalin concentrations (0, 0.05, 0.1, 0.15, 0.2 and 0.25 mL/L) and the EAHAS (0.5, 15, 25, 35, 45 mL/L). For the efficacy bioassay of formalin and Mansoa alliacea "ajos sacha" leaf extract (EAHAS) in controlling monogeneans, a total ...