Résumé : Corrosion of carbon steel is investigated in the presence of Synechococcus sp. (S. sp., an aerogenic strain) with or without the coexistence of Pseudoalteromonas sp. (P. sp., an aerobic strain). Results of electrochemical and weight loss measurements suggest that the corrosion of carbon steel is continuous in S. sp. containing media while it is inhibited initially and accelerated finally in the coexistence of S. sp. and P. sp. This variation in corrosion rate is believed to be closely related to the difference in the growth and metabolism of two strains in different media. In media containing S. sp. alone, the slow reproduction of S. sp. leads to the relatively stable dissolved oxygen (DO) concentration and the continuous corrosion. In the case of S. sp. + P. sp., P. sp. consume DO and form protective biofilm at the initial stage, and then the death of P. sp. and the rapid reproduction of S. sp. cause the increase of DO concentration and the formation of porous corrosion products, leading to the change of corrosion rate.