Isolates of Aquaspirilluni sp., Pseudomonas sp., and Streptococcus sp. recovered from epizootic ulcerative syndrome (EUS)-affected snakeheads (Ophicephalus striatus) in Thailand as well as an isolate of Aeromonas hydrophila recovered from EUS-affected snakeheads in the Philippines were characterized and identified. Each isolate was injected intramuscularly (IM) into healthy catfish (Clarias batrachus) and snakeheads (O. striatus). Results showed in tests with C. batraclius that 24 h after injection, Aquaspirillum sp., Pseudomonas sp., Streptococcus sp. and A. hydrophila induced slight, slight, moderate and severe dermomuscular necrotic lesions, respectively. Among O. striatus, only A. hydrophila induced severe lesions. Streptococcus sp. induced slight lesions 2 days post-injection which healed rapidly, while Aquaspirillum sp. and Pseudonionas sp. did not manifest any dermal lesions. Experiments indicated that among the four EUS-associated test bacteria, A. hydrophila was the most pathogenic, inducing severe dermomuscular necrotic lesions in intramuscularly injected catfish (C. batrachus) and snakeheads (O. striatus). Differences in the susceptibility of O. striatus and C. batrachus to Aquaspirillum sp., Pseudomonas sp. and Streptococcus sp. were evident. Furthermore, this is the first evidence of the association between Aquaspirillum sp. and diseased fish.