We evaluated WNV exposure within the same cohort, as well as a geographically distant cohort of whales by using serologic testing. All testing was performed at the same laboratory by using a standard plaque-reduction neutralization test. In this assay, a 90% neutralization cutoff was used (6) . A 90% plaque-reduction titer >10 was considered positive. Serum from the affected whale and 5 cohort killer whales from the same marine park in San Antonio as well as 5 whales housed at another facility in Orlando, Florida, USA, were evaluated. In each facility, the animals have regular contact with each other. The facilities are geographically separated so the animals do not have exposure to those in the other park. All 6 animals from Texas had 90% plaque-reduction titers >10, ranging from 40 to 80. The 5 whales housed together in Orlando had no measurable titer.