Petroleum reservoir souring, caused by microbially induced production of hydrogen sulfide and other sulfur compounds, and the attendant corrosion are remediated by isolating bacteriophage(s) specific for the problematic bacteria (target bacteria) and adding an effective amount of such bacteriophage(s) to water introduced into or resident in the reservoir to kill at least some of the target bacteria. Suitable virulent bacteriophage(s) may be indigenous to the water, located in surrounding areas, or taken from a known banked stock. Means of concentrating solutions of bacteriophage(s) are also disclosed.