Although a good deal has been written about East Coast Fever in cattle, the literature relating thereto contains very little direct information regarding the parasite which stands in causal relation to the disease. Robert Koch (1898), who was the first to observe the parasite in cases of East Coast Fever occurring in German East Africa, regarded it as but a variety of Piroplasma bovis (= bigeminum) and described the disease as Texas Fever. Theiler (1904) was the first to distinguish clearly East Coast Fever from Redwater. He stated that “the disease has nothing to do with Texas Fever or Redwater; it is a new disease due to a parasite different to the one found in Texas Fever.” Koch (1903—1904), who gave the disease its distinctive name, reached the same conclusions as Theiler. The investigations of Theiler (1904) established the following facts: Cattle which are immune to Redwater are susceptible to East Coast Fever. East Coast Fever is not communicable by blood inoculations (30 experiments, wherein 5 to 2000 c.c. of East Coast Fever blood were inoculated). He noted the absence of haemoglobinuria in the majority of animals affected with East Coast Fever, its presence in the majority of the animals affected with Redwater. He found that in most cases of East Coast Fever, there was no appreciable decrease in the number of red blood corpuscles, this being in marked contrast to what is observed in Redwater. Theiler noted that cattle might harbour both the parasites of Redwater (P. bovis) and those of East Coast Fever (bacillary forms = T. parva). The former generally appeared in the blood “only towards the end of the fever reaction in East Coast Fever,” being previously latent in the animals which had been “salted” against Redwater. He distinguished “two groups of piroplasmosis,” the inoculable (Redwater, canine and equine piroplasmosis) and the uninoculable (East Coast Fever) by injection of infected blood. The parasites in the latter are much smaller than in the former. He named the parasites of East Coast Fever Piroplasma parvum. Theiler distinguished the parasite of East Coast Fever from P. bovis because of the frequent occurrence of bacillary forms and the minute size of the parasite, but he nevertheless retained the new parasite in the genus Piroplasma.