The aim of the present study was to explore the etiology of mortal ulcerations in "capitán de la sabana". Fishes were captured from Fúquene Lake, Cundinamarca department, Colombia (05°28'N 73°44'W) and stocked in 100 L glass tanks in captivity. Ten fishes with skin ulcers were kept in an independent tank and changes in morphology, bacterial flora and organization of tissues were evaluated by means of bacteriology and histology techniques during eight days. Cultures for bacteria were done from the skin ulcers and from the tank water, and an antibiogram was carried out. The analysis showed the presence of common bacteria in the water flora and ulcers: Pseudomona spp. and Escherichia coli, resistant to the amoxicilin and sensitive to the ciprofloxacin, norfloxacin, furazolidon and tetraciclin. The histology demonstratednecrosis of the epidermis and underlaying tissues with inflammatory cells, and gill hyperplasia. Pathological signs are associated with stress factors. It is suggested that aditional studies should be performed on the etiology of the disease and to develop prophylactic activities in order to avoid losses of the fish in captivity which exceed 50% of the fish stock.