Although our 3 bacterial isolates (1D, 3HP and 5HP) and one reference isolate (China) that caused AHPND in this study were identified as V. parahaemolyticus by commonly recognized methods, differences in the rate of mortality and shrimp behavior caused by them not only suggests that they differed in virulence but also reflects the variation among these bacterial isolates. Unknown variables such as shrimp health status, gut microflora of individual shrimp, etc., may increase or decrease the virulence of AHPND isolates. LD50 comparisons from experiments using isolates under different environmental conditions and in combination with other bacterial isolates would be needed to measure such parameters.