Sulfate-reducing bacteria (SRB) cause souring (the reduction of sulfate to sulfide) and associated corrosion in oil and gas fields. SRB monitoring involves the use of most probable number (MPN) methods in which a sample (1 ml) is subjected to serial dilution in glass vials with 9 ml of anaerobic medium, containing lactate and sulfate. This assay can be conducted on-site by field personnel and is routinely used to determine, for instance, the efficacy of a biocide application. In the laboratory, MPNs are best determined by using microtiter plates, which are incubated in an anaerobic hood. Because the dsrAB genes for dissimilatory sulfite reductase, which catalyzes the final step in the sulfate reduction pathway, are highly conserved, conserved primers have been designed to amplify the dsr genes by PCR. These primers (DSRp2060F and DSR4R) are able to generate mixed PCR products reflecting the diversity and/or numbers of SRB in environmental samples. Although routinely used for research purposes, these methods are not yet used widely in the oil and gas industry to assess the presence of SRB and the success of mitigation measures.