Postlarval and juvenile stages of four species of western hemisphere penaeid shrimp (Penaeus aztecus, P. duorarum, P. setiferus, and P. vannamei) were experimentally challenged with white spot syndrome virus (WSSV) and yellow head virus (YHV) isolates originating from Asia. Challenge exposures were accomplished by feeding minced tissue from WSSV- or YHV-infected shrimp tissues. The WSSV challenge of postlarval shrimp resulted in severe infections in P. setiferus and P. vannamei and less severe infections in P. aztecus and P. duorarum. The WSSV challenge of juvenile shrimp(similar to 1 g) resulted in severe infections and 100% cumulative mortality in P. setiferus and P. vannamei, moderate infections and 27% cumulative mortality in P. aztecus, and no signs of infection and 0% cumulative mortality in P. duorarum. The YHV challenge caused serious disease and mortality in juveniles of all four species, but postlarval shrimp appeared resistant to YHV because no virus-related signs of infection, mortality, or distinctive histopathology were detected. The findings of this preliminary study suggest that Asian WSSV and YHV are potentially serious pathogens for the species of western hemisphere penaeid shrimp that were tested.