The Dayet Er-Roumi (classified as a Site of Biological and Ecological Interest) is the only permanent natural lake of the region of Khemisset, Morocco. This lake is interesting however it is a poorly known freshwater fish resource. An ichthyological study was carried out during the year 2019, with the aim to characterize the populations, identify and determine the level of species diversity using synthetic indices (the specific richness, the diversity of Shannon, and the equitability). The objective of this study was to know the current state of the fauna resource and assess the impact of anthropogenic pressure, particularly fishing pressure on this resource. The inventory and identification of the fish made it possible to establish a list of seven species, three of which are indigenous:  Barbus sp., Tilapia Zilli and Anguilla, while the other four species are introduced with the predominance of the species Gambusia affinis and Cyprinus carpio carpio in terms of numbers. They represent respectively 33,7% and 21.6 % of the individuals captured. The other species are comparatively moderately to poorly- represented. The calculation of the synthetic indices showed a non-stable state of the population of Lake Dayet Er-Roumi due mainly to a low diversity with the dominance of some species over others. Measures must be taken to better conserve this ichthyological richness.