Effects of contaminants in fishes might be manifested in many levels of biological organization, including the cause of lesions in gills. This study aimed to identify damages caused by parasites to Centropomus undecimalis’ gills’ tissue structure (Bloch, 1972) and evaluates the quality of fishes’ community in the Ecological Park of Laguna da Jansen, São Luis, Maranhão, Brasil. A number of 33 exemplars of C. undecimalis were used, the first right gill arch was removed from each animal and fixed in 10% formaldehyde, fixed in 10% formalin for 24 hours, afterwards it was decalcified in 10% nitric acid, dehydrated in increasing concentrations of alcohols, diaphanized in in xylene, impregnated and included in paraffin. The 5μm thick cuts were stained with hematoxylin and eosin for histologic description. The parasitological search was performed by light microscopy and the lesions were evaluated through Histological Change Index. From the fishes collected, 48.4% presented parasitic changes associated to structural gills changes, such as epithelial hyperplasia and mucus. It was diagnosed presence of parasites in the gills of the sampled specimens, which belong to three families from the group Monogenea and myxosporean cysts. The Histological Change Index average was 53.3 demonstrating that individuals from species C. undecimalis’s gills collected presented severe tissue modifications. Therefore, the results show that the fishes sampled present a likely imbalanced host-parasite-environmentParasitism 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.