Reverse sample genome probing (RSGP) has been used for the analysis of sulfate-reducing bacteria (SRB) in samples obtained from heavy oil operations in the Wain-wright/Wildmere fields in Western Canada. These reservoirs are shallow (600 m), roughly 25°C, and host a 6% brine. Master filters containing up to 35 independently hybridizing SRB standards were used for analysis of DNA prepared from the samples. The standards represented the diversity of SRB that is now recognized, for example, SRB using lactate, ethanol, benzoate, propionate, decanoate, or acetate as their carbon and energy source. These had previously been isolated from Alberta oil and gas field environments either by single colony purification or by liquid culture enrichment. Twenty four sites were analyzed by RSGP following liquid culture enrichment of SRB on media with one of these six carbon sources. Combination of the results of this method for all enrichments indicated on average the presence of six different SRB at each site. No significant differences were observed in the types of SRB cultured from the two fields. Also, the same SRB standards were recovered by growth of either sessile or planktonic samples. Some oil field production waters were analyzed by direct extraction of DNA from the bacterial population and RSGP analysis. This indicated that the natural population is dominated by lactate- and benzoate-utilizers. Oil fields therefore harbor a variety of SRB. Biocide treatment schemes should ideally take into account this variety and be based on identification of the SRB involved, their known effects on field operations, and their documented biocide sensitivity. In this scheme, RSGP can help in the rapid identification of SRB and others, for example, acid-producing bacteria associated with corrosion.