When stainless steels samples are immersed in natural sea water, an ennoblement of the electrode potential (Esp) up to approximately 400mV vs. SCE can occur, due to the development of biofilm on the samples. The main organisms in the biofilm include bacteria and diatom. In this paper, it was investigated the relationship between the ennoblement of Esp and bacteria species in the biofilm, and the interaction between bacteria and diatom in the Esp ennoblement. To study effect of various bacteria species in the biofilm on the ennoblement of Esp, type 316 stainless steel samples were immersed in 0.4μm filtered natural sea water, where the filtering was expected to restrict bacteria as the only organisms in the water. Under this condition, ennoblement of Esp up to 400mV vs. SCE could not be attained, and the need of other organisms in biofilm, besides bacteria, to promote ennoblement was shown. The analysis of biofilm, formed in natural sea water, on sample with ennobled Esp, detected the presence of sulfate-reducing bacteria, acidic and neutral sulfur-oxidizing bacteria (A-SOB and N-SOB, respectively). It was also observed that the density of N-SOB was high all through the year. To study effect of SOB on ennoblement of Esp, type 304 stainless steel was immersed in solutions containing Thiobacillus thioparus (as N-SOB) and T. thiooxidans (as A-SOB). Remarkable Esp ennoblement was induced by N-SOB, but not by A-SOB. However, as the density of attached N-SOB on the ennobled sample was too much higher than the density to be attained under immersion in unfiltered natural sea water, the ennoblement of Esp was concluded not to occur by that single bacteria. When samples were transferred to diatom containing solution, after immersion in natural sea water for a few days, to promote attachment of bacteria on the samples, the Esp of the samples presented high values, similar to the values observed for samples immersed in natural sea water in summer. After immersion in diatom containing solution, the effect of the density of attached bacteria and diatom on Esp was investigated. For Esp ennoblement up to 200mV vs. SCE, the density of attached bacteria increased up to about 3.5×104pg/cm2, with increasing density of diatom. For Esp ennoblement higher than 200mV vs. SCE, it is suggested that the attachment of diatom on the sample plays a more important role.