In conclusion, we found bacterial isolates that can cause AHPND pathology in specimens from a single Thai shrimp farm exhibiting high, early mortality, and we have shown that these isolates resemble V. parahaemolyticus isolates that were first reported to cause AHPND from Vietnam ( Tran et al., 2013). However, there are indications that our isolates differ in virulence, and we suspect that isolate 2HP may be an extreme example of a variant with relatively low virulence, since it did not cause AHPND histopathology but did give a positive result with a recently introduced PCR method designed for detection of AHPND bacteria. If this turns out to be the case after detailed genetic and molecular analysis, the case definition of AHPND may have to be modified somewhat to accommodate such isolates lest they be overlooked or their importance underrated. Study of such isolates may also be useful in better understanding the basis of virulence in AHPND bacteria.