Environmentally-friendly methods for controlling microbial pathogenesis in aquaculture with probiotic bacteria are becoming increasingly preferred over the use of chemical means, such as disinfectants or antibiotics. Previous research at the Milford Laboratory has shown that naturally-occurring bacteria isolated from the digestive glands of adult oysters (Crassostrea virginica) show promise as potential probiotic additives in oyster larviculture, based on bench-scale experiments. The previous, bench-scale challenge studies reported in the accompanying article (Lim et al. this volume) indicated that 48-h survival of 2-dayold oyster larvae supplemented with Vibrio sp. strain OY15 improved after challenge with pathogenic Vibrio sp. strain B183 compared with the pathogen alone. This study investigated further the effectiveness of probiotic candidate OY15 to improve survival of oyster larvae to metamorphosis under pilot-scale culture conditions, both with and without pathogen B183 challenge. The effective dosage of probiotic candidate OY15 that significantly improved larval survival was determined to be 10³ cfu/mL. The LD₅₀ calculated for pathogen B183 was 9.6 × 10⁴ cfu/mL. Results from these bioassays indicated that addition of probiotic candidate OY15 significantly improved survival of oyster larvae to metamorphosis when challenged with pathogen B183 in pilot-scale trials. These studies can provide the basis for the development of functional foods for use in shellfish larviculture that incorporate a naturally-occurring, probiotic bacterial strain.