The study of the gill fishes of 176 Oreochromis niloticus (L.) was undertaken in the dam of Loumbila (Burkina Faso), in order to obtain the first information on the dynamics of population of the Monogeneans parasites in a population of wild hosts. These data are essential in the search for means to avoid the enormous losses that often occur in intensive pisciculture. This analysis revealed the presence of five species of Monogeneans (Cichlidogyrus tilapiae, Cichlidogyrus halli, Cichlidogyrus thurstonae, Cichlidogyrus rognoni and Scutogyrus longicornis). On the levels of the recorded infestations, these species taken individually probably do not control the population of Oreochromis niloticus. In the dam of Loumbila, the fluctuations of parasitic abundances are neither seasonal nor cyclic. The size either does not have an influence on parasitism. However, the sex effect was noted with C. halli, the females thus were more infested than the males. O. niloticus is parasitized all the year, because the recruitment of these organisms, although relatively weak, is continuous; this logically results in their accumulation in this fish.