The aim of this study was to evaluate the presence of Dawestrema cycloancistrioides in semi-intensive fish farming of fingerlings of Arapaima gigas. Between September and November 2013, 60 individuals of A. gigas born in captivity, were collected in three concrete ponds, from a semi-intensive fish farm in the Peruvian Amazon. For the study of sclerotized structures, parasites were fixed in a solution of ammonium picrate glycerine and mounted in Canada balsam. To visualize internal structures, parasites were fixed in hot formaldehyde solution (4%) for staining with Gomori’s trichrome. The parasitic indexes calculated were prevalence, mean intensity, and mean abundance. This study identified a high infestation of a monogenean D. cycloancistrioides in gills of A. gigas. The prevalence was 100%. The mean intensity and mean abundance of the parasite were 144.9 of parasites per individual. This study confirms the necessity of constant monitoring of fish in order to reduce fish mortality. 1. Introduction The Arapaima gigas is an endemic species of the Basin Amazon and is considered one of the largest freshwater fish in the world. The A. gigas can reach up to three meters in length and 200？kg of total weight [1] and is a much appreciated species with great acceptance in the Amazonian market being regarded as a protein source of the highest quality. However, due to its high nutritional demand for the population of the Amazon region, in recent years the natural stocks of this fish have suffered drastic reduction [2]. Farming paiche is thus a possible solution to the overexploitation of this species in many rivers of the Amazon. However, to allow the breeding, it turns out that the necessity to solve the problems regarding diseases and parasites upsurge, which are affecting this species in controlled environments as a consequence of intensive farming under inadequate management. The dactylogyrid monogeneans are ectoparasites usually attached to the gills of fish. Like other monogeneans, dactylogyrids have a direct life cycle and can easily multiply and disperse under fish culture conditions, reaching very high intensities. These monogeneans have been linked to major losses in fish culture [3, 4]. Recently, studies in several species of farmed fishes in the Amazon region of Peru showed a high infestation by monogeneans species, indicated as the probable cause of high mortality [5–8]. Therefore, with the gradual increase of intensive and semi-intensive fish farming in the Peruvian Amazon, there is a need for constant monitoring of the fish for the diagnosis and
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