ABSTRACT Parasitism in fish can occur due to an imbalance in the environment-host-parasite relationship, usually associated with poor environmental quality. The present study aimed to determine the frequency of parasitism and histological alterations in the gills of Centropomus undecimalis from the Jansen Lagoon Ecological Park in São Luís, Maranhão, Brazil. Thirty-three specimens of C. undecimalis were collected. The first right gill arch was removed from each animal and fixed in 10% formalin for 24 h. These were then decalcified in 10% nitric acid, dehydrated in increasing concentrations of alcohol, clarified in xylene and impregnated with and embedded in paraffin. Five-μm thick sections were stained with hematoxylin and eosin for histologic description and to determine parasite frequency. The gill lesions of each fish were semiquantitatively evaluated by calculating the histological alteration index (HAI). Structural alterations in the gills, such as epithelial hyperplasia and mucus, were associated with parasites in 48.4% of the fish. Three families of parasites from the Monogenean and Myxosporean groups were identified. The HAI average was 53.3, demonstrating that individuals collected from C. undecimalis had severe tissue modifications. Therefore, parasitism caused structural damages in the gill tissues of sampled fish, damages that can impair the organ's physiology, and harm the fish health.