Neobenedenia melleni (MacCallum, 1927) (Monogenea) is a widespread pathogen in marine teleost cultures all over the world. The present paper reports this parasite species in farmed cobia (Rachycentron canadum) in Brazil, for the first time. Some comments on preventive actions for avoiding the disease are made.Keywords: Mariculture, pathogens, ectoparasites. ResumoNeobenedenia melleni (MacCallum, 1927) (Monogenea) é um patógeno amplamente distribuído em cultivo de teleósteos marinhos no mundo. Este estudo relata pela primeira vez essa espécie de parasito em cultivo de cobia, Rachycentron canadum, no Brasil. Comentários sobre prevenção para evitar a doença são discutidos.Palavras-chave: Maricultura, patógenos, ectoparasitos.Research relating to sea-farmed cobia (Rachycentron canadum Linnaeus, 1766) has been increasing recently. The great growth potential of cobia, their easy adaptability and ability to breed in captivity, excellent meat quality and carcass utilization, among other attributes, have induced rapid growth of cobia farming all over the world (CHANG et al., 2007). However, one of the main limitations affecting cobia culture is the diseases caused by helminth ectoparasites (monogeneans) (LIAO et al., 2004), especially Neobenedenia species (OGAWA et al., 2006).Infestations by ectoparasites in farmed fish have been recorded since the late 1950s (OGAWA et al., 1995). Deveney et al. (2001) described an outbreak of Neobenedenia elleni (MacCallum, 1927) parasitizing Lates calcarifer in Australian waters that resulted in the loss of 200,000 fish due to secondary infection. This parasite has been found in many species of ornamental marine fish and sea farming in different countries, thus demonstrating its low specificity to the hosts (BULLARD et al., 2003). In Brazil, Sanches and Vianna (2007) Capsalid monogeneans have a monoxenic, short life cycle, which allows epizootic outbreaks especially in hosts subjected to high stocking densities (THONEY; HARGIS JUNIOR, 1991). It has been demonstrated that water temperature influences the growth, maturity, egg production and infestation levels of Neobenedenia species (HIRAZAWA et al., 2010). These parasites feeds on mucus and epithelial cells of the host, which leads to changes in fish behavior: they drag their bodies against a substrate, such as net cages, thereby causing injuries culminating in secondary infections. Parasitized fish presents lesions in the cornea and skin, blindness, weakness, loss of appetite, hemorrhage, mucus hypersecretion and death due to secondary infections (OGAWA et al., 1995;HIRAYAMA et al., 2009). Our purpose in this paper was to document the first occurrence of capsalid monogeneans in cobia (R. canadum) farmed in Brazil. We have also suggested procedures that can be adopted for disease prevention.Ten juvenile specimens of R. canadum, with mean weight 317 ± 155 g, coming from floating net cages of a marine farm in Ilhabela, São Paulo, Brazil, were sent to our laboratory in March 2010, for diagnostic purposes. The fish were stocked ...