Colonization of the human intestinal tract by a newly proposed species, K. terrigena, was investigated. 5377 different stool specimens from healthy persons (food handlers) yielded 50 isolates (0.9%). Biochemically, low frequencies in the degradation of urea, dulcitol, and utilization of citrate at 37 degrees C were found when compared to K. pneumoniae. At 30 degrees C, urea hydrolysis was observed twice as often as at 37 degrees C. Apart from ampicillin, K. terrigena was susceptible to 12 other antimicrobial drugs tested. Multiple drug resistance was rare, few isolates being resistant against 2-4 antibiotic agents. Capsule typing revealed 30 different serotypes, K 70 and K 14 were the most frequent. Six strains expressed capsule types K 2 and K 5, which have been reported to be associated with virulence in K. pneumoniae. A possible pathogenic role of K. terrigena is discussed.