This study was carried out to determine the reason of mortalities in humpback red snapper (Lutjanus gibbus) and golden trevally (Gnathanodon speciosus) kept in a marine aquarium in Turkey. Moribund fish showed diffuse haemorrhages on upper and lower jaws, superficial and ulcerative skin lesions, tail rot and pale gills. Internally, clear fluid in peritoneal cavity and pale liver were observed. The isolated bacteria were identified as Vibrio harveyi according to their morphological, physiological, biochemical characteristics and API 20E results. These bacterial identifications were also confirmed with 16S rRNA gene sequence analysis. The isolates were determined to be sensitive to sulphamethoxazole/trimethoprim, florphenicol and flumequine and were resistant to erythromycin, kanamycin and ampicillin. Vibrio harveyi infected fish generally showed epithelial hyperplasia in the gill filaments, ulcerative skin lesions, degeneration or necrosis of the hepatic cells, kidney tubules and parenchyma cells of the spleen, multifocal melanomacrophage centres in the kidney and reduced hemopoietic tissue in the spleen.