Case report Eris Jang1, So Min Kim2, Junhyup Song2, Le Phuong Nguyen2,3, Hyukmin Lee2 1Sprayberry High School, Marietta, USA, 2Department of Laboratory Medicine and Research Institute of Bacterial Resistance, 3Brain Korea 21 plus Program for Medical Science, Yonsei University College of Medicine, Seoul, Korea Corresponding to Junhyup Song, E-mail: llive@yuhs.ac Ann Clin Microbiol 2021;24(2):61-65. https://doi.org/10.5145/ACM.2021.24.2.4Received on 20 April 2021, Revised on 20 April 2021, Accepted on 20 April 2021, Published on 20 June 2021.Copyright © Korean Society of Clinical Microbiology. Abstract Roseomonas aerofrigidensis is a gram-negative, strictly aerobic, non-motile bacterium, which was first isolated in 2017 in South Korea. We present the first report of the isolation of R. aerofrigidensis from the peritoneal fluid of a 38-year-old woman with a history of metastatic gastric cancer with peritoneal carcinomatosis. The isolate was resistant to cotrimoxazole. Further research on clinical and microbiological responses to several antibiotics are warranted. Keywords Gastric Cancer Patient,