Technical Abstract: Pseudomonas fluorescens S11:P:12 (NRRL B-21133) is a biological control agent able to suppress several potato diseases and sprouting. Notably, it produces a polysaccharide during liquid cultivation; and the objective of this work was to determine the role of this material in the bio-control process. First, the polysaccharide was isolated, purified, and identified as marginalan, which accumulated to ~3.3 g/L in cultures. The bioactivity of isolated marginalan applied alone or in combination with washed cells of S11:P:12 was tested in potato bioassays of dry rot suppressiveness and sprout inhibition. Since the formulation and storage of a dried biocontrol product is preferred for commercial use, the impact of marginalan on cell survival during drying and storage was also studied. Washed bacteria formulated with 0-6.6 g/L polysaccharide were either applied to HyFlo granules, then slowly dried 24 h with airflow at 50-60% relative humidity, or in 1 µL droplets placed in replicate wells of a micro-plate, then quickly dried 1 h in a biohazard hood. Both Hyflo and micro-plate dry storage results indicated that marginalan significantly reduced cell death after drying, such that the final stable viable cell density was 2.5 to 5 orders of magnitude greater, respectively, than if no marginalan were included with cells. Marginalan had no significant impact on disease or sprout suppression by S11:P:12, and its main benefit to biocontrol was viable cell preservation during drying and storage. When marginalan was formulated with other selected biocontrol strains, its benefits to drying and storage survival were again evident, though more subtle than for S11:P:12; and dry rot suppression was not impacted.