As it is now generally admitted that one or more of the species of fleas occurring on the common rats, Mus rattus and Mus norvegicus, are concerned in the transmission of plague, the study of these fleas has become a matter of the highest importance in connexion with the prevention of the disease. The first point that demands elucidation at the hands of medical entomologists is the identity of the species that is or are responsible. When this point has been satisfactorily determined, attention can be directed to the detailed study of the bionomics of the obnoxious species, as a preliminary to the adoption of appropriate remedial measures. It will be obvious that a sine qua non to a successful attack on the problem is the ability of the investigator to discriminate the various species that he is likely to meet in his researches, and it is the object of the author of these notes to supply a key to the identification of the species of Xenopsylla that occur on rats in India.