The O-serotyping scheme for Providencia was tested on Providencia alcalifaciens isolates collected mostly from two hospitals. The specificites of the somatic (O) antigens of P. alcalifaciens were found to be different from those of Providencia stuartii, and separation of the Providencia typing scheme to allow separate typing of each species led to more efficient typing. All but 4 of 86 isolates were typable. Eighteen serotypes occurred among 53 typable isolates obtained from a pediatric hospital, and 11 occurred among 19 isolates from a general hospital. Thirty-two percent of the isolates from the pediatric hospital belonged to serotype O3, the most frequently isolated and most widely distributed type. The use of the serotyping scheme for P. alcalifaciens is advocated for further studies to examine strains of the species for enteropathogenic types.