Abstract: A microbial system is provided for selective plugging of permeable regions of a subterranean formation, for use in conjunction with injection of surfactant-containing foams. Bacteria indigenous to the target formation are isolated, and selected for ability to degrade the surfactant of interest. Small, non-adherent ultramicrobacteria, or UMB, are prepared from the selected culture by starvation. The UMB and the surfactant-containing foam are then injected into the target formation. The surfactant allows the foam to penetrate into the formation. The UMB then revive to their vegetative state, degrade the surfactant and produce exopolymer, thus plugging the formation.