The Moroccan coast has a rich environmental and ecological heritage and varied as its intrinsic value as its demographic and socio-economic interest. It has a rich and varied environmental heritage (flora and fauna, landscape and natural sites such as dunes and wetlands), which provides resources to the basis of various economic activities (forestry, fishing, grazing and forestry activities .. .). This explains the human and urban concentration, becoming more diffuse, supported and dense on these spaces. This urbanization and densification of the coast irretrievably leads to increased pressure on coastal and problems to confront: solid and liquid waste management, flow (road traffic, drinking water system, energy service ... ), spatial planning (intensive industrialization, coastal development, tourism projects of great effect ...), preserving the landscape and natural resources. The use of bivalve mollusk (Mytilusgalloprovincialis) as a bio-indicator of marine ecosystem pollution provides an advantage of monitoring the coastal contamination and the evolution of the quality of coastal marine species of Rabat-Salé. The objective of this study is a comparison of the dynamic structure of the population of Mytilusgalloprovincialis on the coast of Salé-Rabat (with stations S1, S2 and S3 for Sale and R1, R2 and R3 for Rabat) in us based on the study size frequency histograms to assess the quality of the aquatic ecosystem. Following the results of this study the spatial distribution of the demographic structure of Mytilusgalloprovincialis fluctuates widely from one area to another minimum density of 624 individuals/m2est observed in Rabat and a maximum of 4224 individuals / m2 of Salé. It seems that it depends mainly on the physical and chemical factors, edaphic and hydrological coastline are: the nature of the substance, the tidal phenomenon, the availability of food and behavior of species. Different regression lines show that there is good correlation between the total length to the width of the shell, with significant correlation coefficients (R) of between 50% and 97% Salt and it is greater than 96% for Rabat.