The disease which is the subject of this paper is one of a group of diseases affecting different animals, and caused by parasites possessing very similar characters. The diseases produced by parasites of the genus Piroplasma appear to closely resemble each other, and for this reason it is well to follow the terminology adopted by French investigators in speaking of any one of the diseases in question as a Piroplasmosis. Not only are the parasites similar in these diseases, but they also appear, in all cases where the subject has been worked out, to be transmitted by species of Ixodoidea or Ticks.