Little information is available about natural populations of fishes exposed to polluted waters in Brazil. Contaminants may be lethal or predispose fishes to sickness, being the analysis of aquatic organisms a good indicator of environmental quality. The violinha, Rineloricaria strigilata (Hensel, 1868), is a common loricariid catfish in the Guaíba lake basin, Rio Grande do Sul, Brazil, and has been selected as a bioindicator for being resident, occurring in more than 50% of the samples, and by showing externally detectable neoplasies. These are characterized by its dark pigmentation, bony plates hypertrophy, and hypertrophy and irregular arrangement of the odontodes. Histology showed hypertrophied epitelial tissues, with an increased number of secretory cells, blood cells, and melanina pigments, giving the dark appearance to the neoplasm. Observed frequencies of neoplasm were higher in lake Guaíba than in comparativo samples. Inside lake Guaíba, a pattern of distribution of neoplasm was observed, with the absence of neoplasm in fishes at the collected site that receives water from rio Jacuí, and frequencies of 3.04 to 16.81% in the collection localities in front of Porto Alegre and downstream localities, that receive water from rio Jacuí, rio dos Sinos and rio Gravataí, that drain urban and industrial area of Porto Alegre and neighborhood cities.