The genus Citrobacter is a distinct group of human pathogens comprising three species: Citrobacter freundii (biotypes a and b), Citrobacter amalonaticus, and Citrobacter diversus. In this review the clinical and microbiologic experience during 1972-1978 at the Seattle Veterans Administration Medical Center (Seattle, Wash.) with 298 isolates of Citrobacter is analyzed in relation to a survey of the literature. The most common sources of citrobacter isolates were urine, sputum, and soft tissue exudates. Members of this genus can cause neonatal meningitis and, perhaps, gastroenteritis in both children and adults. Although deep tissue infections due to Citrobacter have been reported only occasionally, in this study a large number of cultures of peritoneal fluid and bone contained Citrobacter. Most isolates of Citrobacter were from elderly, debilitated patients and either represented secondary infections or were of indeterminate clinical significance.