To determine the feasibility of improving oil recovery and the economics of microbial enhanced waterflooding in mature oil wells in the United States, two field pilots have been conducted. Candidate fields were screened to determine whether they have any potential for a microbial system developed at the National Institute for Petroleum and Energy Research (NIPER), and microbial compatibility tests were conducted in the laboratory to select the target field. A specific microbial formulation was selected that was compatible with the chosen reservoir environment and had been shown to recover oil after waterflooding in Berea sandstone and field core. The microbial formulation was designed to improve microscopic oil displacement efficiency by surfactant, gas and acid production from fermentation of molasses. A 20-acre pilot test was initiated in October 1986, and completed in December 1989. Results from this pilot demonstrated that microorganisms could be injected into an ongoing waterflood and that such injection could increase oil production by at least 13%. A larger test (520 acres) was completed in the same formation to evaluate the feasibility of commercial application of the technology. This field pilot was injected with microorganisms and molasses from a centralized injection station in June 1990. Although microorganisms were injected only once per site, nutrient injection continued throughout the project life. All 19 injection wells were treated, and oil production was monitored from the 47 production wells. Injection pressures and volumes were monitored throughout the project. No operational problems were encountered. At the end of May 1993, oil production was improved by 19.6 %. Results from both projects are presented and the potential for microbial-enhanced waterflooding technology is evaluated.