Hatchery production of shellfish seed is necessary to supplement natural recruitment, which is constrained by various stresses, including habitat loss, pollutant contamination, overfishing, and climate change. Bacterial diseases are considered to be a major cause of mortality in hatchery shellfish larviculture; however, overuse of antimicrobials can result in development of resistant strains of bacterial pathogens. The use of probiotics for disease prevention and improved nutrition in aquaculture is becoming increasingly popular as the demand for environmentally-friendly aquaculture grows. The objective of this study was to isolate and evaluate the efficacy of new probiotic bacteria that, incorporated into functional foods for use in shellfish hatcheries, may significantly improve larval survival. First, 26 probiotic-candidate bacteria were isolated from oysters, scallops, and a mass culture of green algae. Fifteen of these isolates (8 oyster strains and 7 bay scallop strains) inhibited known scallop-pathogen bacterial strains B183 and B122 in disk-diffusion assays. Similar to control (unchallenged) oyster larvae, survival of oyster larvae exposed to these 15 probiotic candidates for 48 h was more than 90%. The probiotic candidates were then reisolated from challenged larvae and characterized by Gram stain, colony morphology on solid agar, and the Biolog Bacterial Identification System, finding only 7 distinct strains. Using 12-well microplate assays, 5-day challenges were performed to confirm positive effects of these 7 probiotic candidates on larval survival when challenged with pathogen B183. Oyster larvae exposed to probiotic candidate OY15 had the highest survival; furthermore, survival of pathogen-challenged larvae was significantly improved by the presence of OY15 compared with pathogen alone. In addition, probiotic candidate OY15 exhibited no toxic effects on the microalgal feed strain Isochrysis sp. (T-ISO) in the range of 10²-10⁴ cfu/mL. Future studies will confirm optimal dosage and positive effects of probiotic candidate OY15 on survival during long-term rearing of oyster larvae.