Parasites of the genus Clinostomum complanatum (Rudolphi, 1814), colloquially known as white grubs, are cosmopolitan parasites of fish-eating birds, freshwater snails, fish and amphibians. The first case report of Clinostomum on human infection was seen in 1995 in Korea, it was considered a zoonotic parasite called yellow group diseases; this parasite causes significant damage to fish farms. This parasitic species has been observed in recent years in Japan, China, and Iran to the west, Taiwan, Turkey; and began to be reported from country to country. Thousands of specimens of Tilapia fish (Oreochromis niloticus, Linnaeus, 1758) from the ISET experimental station in Rosso were examined for the Clinostomum complanatum parasites. Parasites taken directly from the attachment sites on the fish are fixed in 70% alcohol and stained with acetic carmine. The number of parasites found was recorded per fish. The sex, total length and weight of the fish are also recorded. The parasites were mainly found attached to the tissues behind the oral cavity, gill cavity, and eyes. 253 Fish were found infested with six hundred and sixty-one (661) parasites; the results obtained showed a prevalence of 29.05% greater in females than 22.30% in males. The females are more vulnerable to being infested with roughly an intensity of 2.93 parasites per fish while males carry less than 2.27 parasites. The infestation of farmed fish by this freshwater parasite moved for the first time in Mauritania, is a limiting factor in the increase in farm productivity in the country.