Natural transmission of the epizootic ulcerative syndrome (EUS) was conducted on naïve snakeheads Ophicephalus striatus (also known as Channa striata) kept (A) in aquifer water, (B) in lakewater, (C) cohabiting with EUS snakeheads in lakewater, and (D) cohabiting with apparently healthy snakeheads in lakewater during the 1994 to 1995 EUS season. The results showed that EUS-like lesions developed in 6 to 14 d among naïve snakeheads cohabiting with EUS snakeheads and with apparently healthy snakeheads in lakewater (Treatments C and D). Among naïve fish exposed to lakewater (Treatment B), similar lesions developed in 16 to 21 d, while naïve fish in aquifer water (Treatment A) did not develop EUS-like lesions. EUS signs began as Grade I (slight) lesions that gradually progressed to Grades III-IV (severe) 3 to 5 d from lesion onset, similar to the naturally affected EUS fish. The virus was recovered from some but not all naturally EUS-affected snakeheads, snakeheads with healing lesions and apparently healthy snakeheads, but not from naïve snakeheads. The results provide evidence of a waterborne horizontal transmission of the EUS-associated virus. This is the first report of a successful horizontal transmission of the EUS-associated virus from apparently healthy snakeheads to naïve fish under natural conditions and of virus recovery in tissue culture from naturally exposed experimental fish.