Riemerella anatipestifer (RA) infection is a contagious disease of domestic ducks, turkey, and various other birds. In a flock of mallard ducklings, about 30% of the birds, 3 weeks old, showed lethergy, greenish diarrhea, tremor of head and neck, and died 2-3 days after signs appeared. Grossly, fibrinous exudates covered the heart and surface of the live. Microscopically, mononuclear cells and heterophils were primarily inflammatory cells in the exudate. These were also observed in the meninges in brain. Microbiologically, gram (-) short rod bipolar shaped bacteria were recovered on blood agar and agglutinated by antisera of R anatipestifer. Sulfamethoxasole/trimethoprim were administered and clinically effective. This case was a R anatipestifer infection caused fibrinous pericarditis, hepatitis and meningitis in mallards.