In vitro and in vivo activities of a biocontrol agent, Serratia plymuthica strain A21-4, was evaluated for the control of Phytophthora blight of pepper. Strain A21-4 inhibited mycelial growth, germination of zoosporangia and cystospores, and formation of zoospore and zoosporangia of Phytophthora capsici in vitro. In the pot experiment, incidence of Phytophthora blight of pepper in non-treated control was 100% at 14 days after inoculation, while no disease was observed in the plot treated with S. plymuthica A21-4. In the greenhouse test, infection rate of pepper in the non-treated plots was 74.5%, while it was only 12.6% in the plots treated with A21-4. Results indicate that S. plymuthica A21-4 is a potential biocontrol agent for Phytophthora blight of pepper.