The nematode Goezia spinulosa (Diesing, 1839) (Raphidascarididae) is redescribed based on specimens found in the stomach and intestine of the naturally infected arapaima Arapaima gigas (Schinz) from the Mexiana Island, Amazon River Delta, Brazil. Light and electron microscopy examinations revealed some previously unreported or inaccurately described morphological features in the species, such as the position of the excretory pore, phasmids in the male or the number (4) of postanal papillae. The morphology of G. spinulosa is compared with that of other four congeneric species parasitizing freshwater fi shes in South America. This nematode seems to be one of the most pathogenic parasites of A. gigas in the Mexiana Island, which are responsible for a high mortality of cultured arapaima fi ngerlings. Apparently, the source of G. spinulosa infection for arapaima fi ngerlings cultured in tanks was the infected plankton collected in the localities inhabited by wild arapaimas. Therefore, control measures should include the sterilisation of the plankton before its use as food for fi sh. A rare infection of Eustrongylides sp. larvae (Dioctophymatidae) in arapaima fi ngerlings was also found (new host record); the larvae were inside swellings on the body surface.