Females of Rhipicephalus (Boophilus) microplus ticks ingurgitated and in the prepost phase were submitted to doses of 0, 5, 10, 15, 20 and 25 Gy of Cobalt-60 with the aim of evaluating their effects on ovarian morphohistology with consequent establishment of the degree of damage caused to the reproductive organs of this species. For this purpose, 24 hours after exposure to Co-60 (142 Gy/h) the ectoparasites were dissected and had their ovaries removed and prepared for histology with haematoxylin and eosin staining. The results obtained showed that the 5 Gy dose radiation had a morphological aspect similar to the control standard adopted. The histological sections related to doses of 10 and 15 Gy did not cause many changes in the ovaries, except that changes were observed in the calf granules (size, distribution and staining pattern), as well as the extensive presence of cytoplasmic vacuoles in the ovocytes, especially in the region that makes contact with the ovocyte/pedicel, suggesting the occurrence of changes also in the physiology of the organ. In the ovaries exposed to doses of 20 and 25 Gy, severe alterations were observed in the organ as a whole, as well as in the germ cells (oocytes) which suffered alterations in size and shape, distribution of calf granules, involvement of the DNA present in the germinal vesicle (oocyte nuclei), besides the extensive cytoplasmic vacuolization, alterations which made the maturation of these cells impossible and consequently inhibited the production of new individuals.