This work studies the classification, by numerical procedure, of 122 strains belonging or related to the genus Klebsiella. Four classes can be defined: J (= K. pneumoniae: 29 strains for the most isolated from human faeces), K (related to K. pneumoniae and to K. oxytoca: 24 strains isolated from water), L (related to K. pneumoniae although, tetrathionate-reductase+, m-hydrobenzoate+: 37 strains isolated from water or soil), M (= K. oxytoca: 22 strains for the most isolated from water). The importance of the origins of the strains is emphasized.