¹College of Oceanography and Ecological Science, Shanghai Ocean University, Shanghai, 201306, CHINA DOI : 10.9775/kvfd.2025.34626 To determine the pathogen responsible for gastroenteritis mortality in a yellow-margined box turtle (Cuora flavomarginata). A dominant single colony, A1, was isolated from the liver lesions of the diseased C. flavomarginata on the farm. Based on 16S rDNA sequencing and comparative analysis in NCBI, the isolate showed 99.58% similarity to Citrobacter braakii. Antimicrobial susceptibility testing revealed that strain A1 was resistant to multiple antibiotics. The pathogenicity of the strain was confirmed through experimental infection in red-eared sliders (Trachemys scripta elegans), which resulted in clinical signs consistent with gastroenteritis. Histopathological examination of infected tissues revealed inflammatory cell infiltration in the gastric submucosa and lamina propria, as well as degenerative changes in hepatocytes, including cytoplasmic loosening, mild edema, and cholestasis. Additionally, structural alterations were observed in the intestinal mucosa, accompanied by inflammatory infiltration. Our study demonstrates that C. braakii is one of the pathogenic agents responsible for gastroenteritis in turtles. Preliminary discussions were conducted from the perspectives of pathogenicity, antimicrobial susceptibility, and histopathology, thereby providing a theoretical foundation for the prevention of turtle diseases caused by this microorganism. Keywords : Antimicrobial resistance, Citrobacter braakii, Cuora flavomarginata, Experimental infection, Histopathology, Turtle