To preliminarily determine acute CuSO4 toxicity in cachama blanca (Piaractus brachypomus) fingerlings, 110 individuals were exposed to seven different concentrations: 0 (control), 0.25, 0.50, 1.00, 1.50, 2.00 and 4.00 ppm using semi-static system aquariums. Water hardness in reference to CaCO3 was constantly maintained at 20 ppm during the experiment. Individuals exposed to 2.00 and 4.00 pm manifested hyperexcitement stages alternating with lethargy and swimming-axis loss, with 100% mortality at 24 and 6 h respectively. Individuals exposed to 1.00 and 1.50 ppm for 24 h showed cumulative mortality of 55 and 90% respectively. Likewise, these groups presented a moderate increase in mucus production at branchial level accompanied by clinical disnea. Interlamellar hyperplasia and severe lamellar fusion, as well as gill congestion, were observed microscopically in those exposed to 4.00 ppm. On the other hand, interlamellar and interfilamental hyperplasia and inflammatory cell infiltration, principally granulocytes, were observed in fingerlings exposed to 0.50 ppm. Lethal concentration 50 (LC50) at 48h of exposure was 0.94 ppm (0.82-1.08, IC 95%). These preliminary results allow inferring that cachama blanca fingerlings are relatively susceptible to the toxic effects of CuSO4 at the concentrations and conditions described for the present study.