We isolated and screened aerobic denitrifying Bacillus stains with antagonistic effects on Streptococcusagalactiae from the intestinal contents of Nile tilapia (Oreochromis niloticus). Antagonistic ability was detectedusing morphological, physiological and biochemical characteristics, and 16S rRNA gene sequencing. Optimalgrowth conditions and the ability to hydrolyze starch and protein were tested, and drug sensitivity and safety testswere performed. We isolated a strain called NY 5 that was identified as Bacillus cereus. A total of 50 mg/L nitritenitrogenwas removed completely within 12 hours after inoculating 1% NY 5 bacterial liquid on denitrificationperformance testing medium. NY 5 had antagonistic effects on 21 strains of S. agalactiae preserved in our laboratoryfrom different sources. The mean diameter of the inhibition zone was (26.67±3.00) mm. NY 5 grew well attemperatures of 25–40℃, salinities of 0–40, and pH values of 5–9. NY 5 hydrolyzed casein and starch. NY 5 wassensitive to most of the antibiotics tested but was resistant to nine antibiotics such as Penicillin G, maddie mycin,and cefazolin, etc. Tilapia with mean body weight of (6.0 ± 1.1) g did not die or show any symptoms after exposure to2.0 × 107 CFU/mL NY 5 bacteria in the water or after injection with 200 μL of 2.0×106 CFU/mL NY 5. These resultssuggest that the NY 5 strain is B. cereus. This study provides a useful probiotic source to effectively inhibitgrowth of S. agalactiae isolated from tilapia.