Carbon steel coupons were exposed to nutritionally-poor synthetic wastewater inoculated with activated sludge from a municipal waste water plant. Biofilm formation was observed after one day incubation, and the thickness of the film increased proportionally with the incubation period. Mass loss of the coupons was also proportional to the incubation time, and reached 70.4 (mg/cm(2)) after incubation for 140 d. The observed mass loss was 5 times as much as that under sterile conditions. To characterize the microbiologically influenced corrosion (MIC) of carbon steel, structural analysis of the biofilm was performed. Rapid decrease in the dissolved oxygen (DO) concentration in the zone near the surface of the biofilm was observed by a microelectrode mounted on a micromanipulator. Heterogeneous distribution of the DO concentration on the surface of the steel plate was observed after multiple analyses. The heterogeneous structure of the biofilm composed of viable cells, inanimate objects, voids and pores was elucidated by confocal scanning Laser microscopy. Concentrations of both aerobic bacteria and sulphur-reducing bacteria in the biofilm decreased with the incubation time, indicating that the increase in the biofilm thickness reflected an increase in the density of dead microbial cells or in extracellular polymer accumulation by the microbes. The average roughness of the metal surface observed after 112 d of incubation was +/-7.14 mu m, which was 14.1% of the average thickness of the coupons. These observations indicated that uneven distribution of the DO profile and the cell concentration were critical for MIC of the carbon steel.