Seahorse (Hippocampus spp.) are economically valuable species and are used in traditional medicine. In Vietnam, seahorse cultivation has been recently developed in some regions namely Khanh Hoa, Ninh Thuan, Hue, Da Nang and Vung Tau. Some seahorse farms in Khanh Hoa are now facing problems with unknown disease occuring in common seahorses (Hippocampus kuda Bleekers 1852). In the present study, luminous bacteria were isolated from a total of 35 Hippocampus kuda, which had ulcerations. Physiological and biochemical testing as well as 16S rDNA sequencing confirmed that the photobacterium (named TNX-X1) was closely related to Vibrio harveyi. Infectivity studies (challenge test) were conducted in Hippocampus kuda showed that this Vibrio isolate caused a disease with symptoms including white changing, viscous skin and ulceration. LD50 (Lethal dose 50%) for fish with an average body weight of 1 – 2g was 5x10⁶ CFUxfish^-1. The isolate, in addition, was resistant to certain antibiotics including ampicillin10µg/disc (A10), cefadroxil 30µg (CD30), amoxicilin 25µg (AMC25) and cefalexin 30µg (CL30) (in total 9 antibiotics).