The chemical and biological specificities of seawater make it an aggressive environment against the corrosion of many materials including steel low or moderate allies. These materials are widely used in the manufacturing of marine port infrastructure. These structures are exposed to two kind of corrosion: general corrosion and localized corrosion. The immersion of a material in seawater provokes the formation of a biofilm containing bacteria and microalgae, where are fixed the macro-organisms, which can also play a decisive role in the corrosion of metallic materials. This work addresses this issue related to the bacterial contamination by sulfite-reducing bacteria of seawater of the Agadir coastal and its potential impact on the resistance of the corrosion of the mild steel. For carrying out this work, we used the microbiological, electrochemical and the surface analysis techniques. We observed that the seawater of Agadir coastal is microbiologically polluted and it has a considerable effect on the corrosion of steel. © 2015 The Authors.