A serotyping scheme for Enterobacter cloacae based on heat-stable somatic antigens is described. A total of 28 antisera were prepared in rabbits, and titers of agglutinins were high (greater than 640). Some cross-reactions were observed, and 11 sera required absorption before routine use. Of 300 clinical isolates from 66 hospitals, 77.6% were typable, 11.4% were not agglutinated by any of the sera, and 11.0% were autoagglutinable in saline. The eight most frequent serotypes were O3 (21.3%), O8 (13.3%), O1 (7.6%), O13 (5.0%), O9 (4.7%), O10 (3.0%), O16 (3.0%), and O25 (3.0%).