The clinical records of 46 patients with intra-abdominal and/or bacteremic infections caused by Citrobacter organisms were reviewed to determine whether the isolation of Citrobacter organisms from these sites could be used to determine its portal of entry. The isolation of Citrobacter organisms from intra-abdominal or pleural fluid cultures was associated with a biliary tract or upper gastrointestinal source in 36 of 41 patients. The biliary tract or small bowel was likewise the portal of entry for 8 to 13 patients with Citrobacter septicemia. Thus, the isolation of these organisms from the blood, pleural space, or abdominal cavity strongly suggests that the upper gastrointestinal (or biliary) tract is the site of significant pathologic disease. Of the antibiotics tested, moxalactam, cefotaxime, and amikacin were most effective against Citrobacter.