The ability of the SRB Desulfovibrio desulfuricans to colonize aerobic heterotrophic or nitrifying biofilms on stainless steel surfaces was investigated by fluorescence in situ hybridization (FISH) in combination with confocal laser scanning microscopy (CLSM), the polymerase chain reaction (PCR) directed to the dissimilatory sulfite reductase and by oxygen microelectrodes. Biofilms of heterotrophic bacteria and of nitrifying bacteria pregrown on steel coupons in laboratory devices were not invaded by cells of D. desulfuricans within I week, even though large pores as possible entry paths and anoxic zones and sulfate in the medium as prerequisites for activity of D. desulfuricans existed in the biofilms. On the contrary, coinoculation of D. desulfuricans and aerobic heterotrophic bacteria from water of a cooling water system led to immediate establishment of a small but detectable population of D. desulfuricans distributed over the whole biofilm depth. The fact that individual SRB attached efficiently and irreversibly to steel suggests that in coinoculation experiments, establishment of SRB in the biofilm possibly occurred from the biofilm base, which is the zone that became anoxic first. Based on these findings it is suggested that the idea of protecting surfaces from SRB-induced corrosion by pregrowing and maintaining specific bacterial biofilms should be further pursued.