The development of Babesia bigemina in Boophilus microplus were studied in experimental conditions, using crossed-breed bovine from free-area of these parasites. Stages of the hemoparasites were observed in the tick vector, starting from the infected red-blood cells observed in the gut of engorged females, from the first 24 hours after detachment to the emergence of sporokinets in the larvas. In the period from 24 to 48 hours after detachment of the engorged females (DEF), the presence of some infected red-blood cells was verified, beside the occurrence of ray bodies and of vermiculars forms, known as oocynets. Since 72 hours after the DEF, the of okinets presence was observed in the cytoplasm of the epithelium cells besides of great sporokinets number in development. At same period, the presence of sporokinets of B. bigemina in the hemolymph samples was observed inside the hemocytes. After the fourth day of incubation beside the presence of the sporokinets was also verified in the Malpighi's tubes and ovaries. As well as in the ticks eggs from the sporokinets were also observed ticks eggs from the fourty day after the natural detachment of the engorged females of the host.