Biofilms were generated on the surfaces of heat‐treated or untreated mild steel coupons by incubating for 4 weeks with or without single or mixed inocula of the anaerobic sulphate‐reducing bacterium,Desulfovibrio desulfuricans, and the aerobic pseudomonad,Pseudomonasfluorescens. P. fluorescensbiofilms were scanty, comprising mainly of approximately circular groups of haematite crystals with bacterial cells (mean 102·67·cm−2) aggregating on and around them. On cleaned coupons, circular areas of corrosion corresponding to the crystalline clumps were observed.D. desulfuricansbiofilms were thicker and more uniform. They were composed of iron sulphide corrosion products, bacterial cells (104·33·cm−2) and large amounts of polysaccharide (EPS), seen in the scanning electron microscope. Mixed culture biofilms appeared similarly thick, but somewhat patchier. Cells ofD. desulfuricansand iron sulphide corrosion products were the main components. Cell populations were 101·69·cm−2forP. fluorescensand 104·21‐cm−2forD. desulfuricans. Somewhat less EPS was observed in the scanning electron microscope.