1.The five strains of rickettsia are discussed and compared according to the criteria suggested by Pinkerton (1936).
2. It· is concluded that they fall into two groups:
(a) Typhus group – endemic, murine or rat typhus.
(b) Rocky Mountain spotted fever group - fievre boutonneuse, strains “Robertson”, “Appleton”, “Hare”, which are similar and show only minor strain differences.
3. A proposed nomenclature is discussed.