Pseudomonas elongata was isolated from naturally infected scattered mirror carp (Cyprinus carpio) and its pathogenicity was tested by intramuscular injection. The infection caused mortality in scattered mirror carp with gross clinical abnormalities such as dark coloured of a location on body surface, cataract in eyes, haemorrhagic damage of liver, irrigation in kidney, anemia, swollen intestine, fins rot and hyperaemia in operculum and skin. Lethal Doses(50) (LD(50)) of Pseudomonas elongata was calculated 2.24 x 10(5). No significant difference was obtained among enumered of pathogenic bacteria isolated from gill, liver, kidney tissues and total pathogenic bacteria. Sensitivities of Pseudomonas elongata against 50 chemotherapeutants were tested. Minimum inhibitory concentrations of enrofloxacin and chloramine T to the isolate were calculated 5 mL L(-1) and 10 mg L(-1), respectively. Best treatment method was no mortality performed with orally enrofloxacin application and chloramine T bath.