The prevalence of Klebsiella species was determined in 25 adults from each of four sections in the Minneapolis-St. Paul area, 100 doctors, nurses, and laboratory technologists from the University of Minnesota Hospitals, 100 patients newly admitted to the hospital, and 100 patients who had been in the hospital longer than 15 days. Klebsiella was isolated from 37%, 30%, 33%, and 43% of these populations, respectively. All but one of the isolates from individuals not associated with hospitals were resistant to only ampicillin and carbenicillin, whereas 18% of strains isolated from patients who had been in the hospital more than 15 days were resistant to several drugs. R-factors were present in resistant bacteria. Fifty-one of 72 possible serotypes of Klebsiella were found, but no types were predominant in any group. Results of biochemical tests were comparable to the results reported by other workers, except that one-third of the Klebsiella organisms isolated were indole-positive.