Résumé : Microbial biofilms that form on metallic surfaces in natural seawater are known to generate efficient oxygen-reducing cathodes. The microbial catalysis of oxygen reduction is a major mechanism of corrosion in marine aerobic environments; it can also be exploited to develop biocathodes for microbial fuel cells. In the latter case, seawater biocathodes have the great advantage of operating in high-salinity electrolytes. Four bacterial strains (Pseudoalteromonas sp., Marinobacter sp., Roseobacter sp., Bacillus sp.) were isolated from an oxygen-reducing biocathode formed in natural seawater. 16S rDNA pyrosequencing analysis showed that the strains isolated were representative of the microbial community that composed the initial multispecies biocathode, which was dominated by Gamma-Proteobacteria. Each strain was able to form oxygen-reducing monospecies biocathodes both in natural seawater and in a synthetic medium with the same salinity. Stable current densities of 40 mA m−2 were produced under constant applied potential (−0.30 V/SCE) and up to 0.8 A m−2 was recorded at −0.60 V/SCE. This work provides the first description of monospecies biocathodes designed with salinity-tolerant strains and offers an experimental model to advance the investigation of the microbiological and biochemical processes on seawater biocathodes in well-controlled conditions.