To examine the activity of bacteria involved in cathodic depolarization and surface corrosion on stainless steel in an in situ model system. The microautoradiographic technique (MAR) was used to evaluate the activity of bacterial populations on stainless steel surfaces with a single cell resolution. Anaerobic uptake and fixation of C-14-labelled bicarbonate occurred within corrosion sites in the absence of atmospheric hydrogen or other external electron donors, whereas it was taken up and fixed by bacteria at all other stainless steel surfaces in the presence of atmospheric hydrogen. This indicates that the bacteria utilized electrons originating from the corrosion sites due to the ongoing corrosion (cathodic depolarization). Under in situ conditions, bacteria were fixating C-14-labelled bicarbonate at corrosion sites in the absence of atmospheric hydrogen. This indicates that electrons transferred to the bacteria provided energy for bicarbonate fixation due to cathodic depolarization. Application of the MAR method showed ongoing biocorrosion in the applied in situ model system and allowed in situ examination of bacterial activity on a single cell level directly on a metal surface providing information about potential corrosion mechanisms. Furthermore, application of fluorescence in situ hybridization in combination with MAR allows for identification of the active bacteria.