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The present study assesses the diversity and seasonal variation of parasites in Oreochromis niloticus (Nile tilapia) cultivated in excavated tanks in the Federal District, Brazil. A total of 120 specimens of O. niloticus were collected in 12 monthly collections. Water quality parameters were checked at all tanks. The animals, sacrificed by immersion into anesthetic solution, had the gills scraped, in which the mucus was analyzed under an optical microscope. Parasitological examination showed Trichodina sp., Epistylis sp., Ichthyophthirius multifiliis, and Monogenea helminths in the fish gills. We found statistically significant differences between Monogenea parasites collected in the fall and winter and between I. multifiliis protozoans collected in fall-winter and winter-summer periods. Except for Epistylis sp., all parasites showed abundance peaks in O. niloticus specimens collected during the winter, which may characterize the seasonality of these parasites in the Federal District region. Monogenea helminths were the most prevalent among the parasites found, with the highest prevalence during the fall. The mean abundance of parasites was similar between the fall and winter. However, the abundance of monogenetic trematodes was higher between the winter and summer. I. multifiliis showed significant variation between fall-winter and wintersummer periods. [ABSTRACT FROM AUTHOR]