Technical Abstract: The interactions between Bradyrhizobium aponicum and rhizobiophages in nodulating diverse soybean genotypes were examined in field experiments in the Delmarva region in the growing seasons of 2001 - 2003. Twenty phabes infecting B. japonicum strains USDA 38, 110, 117, 126, and 147 were isolated from the rhizosphere soils of soybean plants growing in the Delmarva region. Phage isolates produced plaques on host bacteria that ranged in diameter from0.5 to 2.0 mm. These plaques also varied in their morphologies and characterization. The field experiments showed that strain TA11NOD- was the best microsymbiont with both the forage soybean cv. Tyrone and Roundup Ready cv. AG 4602, producing 309 and 303 nodules plant-1, 1.4 and 3.34 umol C2H4 plant-1h-1 and 2.2 and 3.8 t ha-1 of seed yield, respectively. Selected phages virulent to ineffective B. japonicum strains were used to reuce the competitive ability of these indigenous strains. Nodule occupancy by undesirable indigenous strains was reduced by 44 percent, thus the nodule occupancy of the effective inoculant strain was increased by 55 percent. This study indicates that the use of rhizobiophages as biocontrol agents requires the identification of symbiotically competent, phage resistant bradyrhizobia with the ability to promote soybean growth and yield of their specific hosts.