Despite its relatively limited corrosion resistance, carbon steel is one of the most widely used engineering materials. An effective technique to improve its corrosion resistance, particularly during transport and storage, is phosphating. On the other hand, it is well known that different environmental bacteria can grow and develop on metal surfaces, forming biofilms. Some of those bacteria are capable of solubilizing phosphate. The objective of this work was to evaluate the stability of iron and manganese phosphate conversion coatings applied on SAE 1010 carbon steel in the presence of biofilms for two environmental bacterial strains: Pseudomonas aeruginosa and Paenebacillus sp. Both strains are biofilm-forming and phosphate solubilizing bacteria. Electrochemical techniques complemented with surface observation by scanning electron microscopy were applied. The electrochemical results showed that, although the presence of a bacterial biofilm in most cases produced a decrease in the corrosion rate, they provoked a shift of the pitting potential to more active values. This shift led to narrower passive zones, compromising the protection provided by the phosphate conversion coating to the 1010 SAE carbon steel by facilitating the onset of localized corrosion. The degree of the compromise of passivity was dependent on the coating and the bacterial strain. P. aeruginosa biofilms were more detrimental for both coatings than the Paenabacillus sp. biofilms. Thus, it is important to point out that to maintain the corrosion protection imparted by phosphate conversion coating to carbon steel, it is necessary to avoid the development of biofilms on the coated surface.