A novel interdomain consortium composed of a methanogenic Archaeon and a sulfate-reducing bacterium was isolated from a microbial biofilm in an oil well in Cahuita National Park, Costa Rica. Both organisms can be grown in pure culture or as stable co-culture. The methanogenic cells were non-motile rods producing CH4 exclusively from H-2/CO2. Cells of the sulfate-reducing partner were motile rods forming cell aggregates. They utilized hydrogen, lactate, formate, and pyruvate as electron donors. Electron acceptors were sulfate, thiosulfate, and sulfite. 16S rRNA sequencing revealed 99% gene sequence similarity of strain CaP3V-M-L2A(T) to Methanobacterium subterraneum and 98.5% of strain CaP3V-S-L1A(T) to Desulfomicrobium baculatum. Both strains grew from 20 to 42 degrees C, pH 5.0-7.5, and 0-4% NaCl. Based on our data, type strains CaP3V-M-L2A(T) (= DSM 113354( T) = JCM 39174( T)) and CaP3V-S-L1A(T) (= DSM 113299( T) = JCM 39179( T)) represent novel species which we name Methanobacterium cahuitense sp. nov. and Desulfomicrobium aggregans sp. nov.