Plesiomonas shigelloides was isolated from blood culture of a 53-year-old man with fever, who had treatment history of gastrointestinal malignancy. The patient showed neither clinical features nor hematological finding which suggest bacteremia. Identification of the isolate was delayed because of its similar characteristics with Aeromonas spp. and other gram-negative bacilli. The isolate was misinterpreted as susceptible to ampicillin by the first disk diffusion test. It may not always easy to identify P. shigelloides by conventional tests and to determine its antimicrobial susceptibility accurately, as laboratorians rarely have experience with the organism and as the organism may show unusual inhibition pattern when tested by disk diffusion method or Etest. (Korean J Clin Microbiol 2000;3:137-141)