A case of ocular diplostomiasis (parasite cataract in fish or eyefluke disease) is described for the first time in a pisciculture with earth ponds in the province of Chubut. Snails (Chilina dombeiana) were analysed by squashing and rainbow trout (Oncorhynchus mykiss) by necropsy with microdissection of eyes, for the diagnosis. The findings of sporocysts of Diplostomidae in the hepatopancreas of snails and metacercariae of Diplostomum sp. in the lens of fish confirm the cataract as parasitic. Detailed recommendations for the control and prevention of the disease, based mainly on ecological methods, taking into account the cycle of the parasite and the characteristics of the building of the pisciculture are given. This paper amplifies the distribution of this disease in cultured rainbow trout, including the southern part of South America.