Two dogs were infected with Babesia canis by the bite of 1500 and 100 adult Rhipicephalus sanguineus respectively. Both dogs showed blood para¬sites after a prepatent period of 6 days. Through the prepatent period 5 ml of blood were taken daily from each dog and inoculated intravenously into 10 puppies. From the latter, those which were inoculated on the 1st, 2nd and 3rd day of the prepatent period remained negative, while those inoculated on the 4th and 5th days became infected with Babesia. In accordance with the knowledge obtained regarding the preervthrocytic development in the other hemosporidia, the results of our experiments may be interpreted as indicating the existence of an early developmental phase of the babesia in tissue cells of the vertebrate host, corresponding to a negative phase of the blood, and of at least 3 days duration.