One of the objectives of the microbial research at NIPER has been to identify possible environmental hazards associated with applying microbial enhanced oil recovery (MEOR) processes in the field. Previous laboratory studies have involved the use of two microorganisms, Bacillus species and Clostridium species, which are used frequently for MEOR field projects. The ability of these two bacterial systems to coexist and perform the desired MEOR process in the presence of indigenous reservoir bacteria has been evaluated and reported earlier. This study focuses on the compatibility of Bacillus and Clostridium spp. with sulfate-reducing bacteria present in porous media. Experiments were performed in Berea sandstone cores to examine what effects the growth and survival of Desulfovibrio desulfuricans, a sulfate reducing bacterium, had on the survival and ability of Bacillus and Clostridium spp. to recover incremental residual crude oil. Results based on the core studies have shown that Bacillus species survive and overgrow the Desulfovibrio sp. more consistently than the Clostridium species. In addition, the ability of Bacillus to recover residual oil is less diminished by the Desulfovibrio than it is for the Clostridium species. Many of the core results indicated that the injected bacterial systems can overgrow and eliminate the growth of the sulfate-reducing bacteria. This finding is of importance to MEOR applications because the reduction of sulfate-reducing bacteria by the injection of microbial strains could aid in solving the problems of corrosion and well damage in oilfield operations.