Bos taurus indicus females provide a greater in vivo oocyte recovery (2-4 times more) in comparision with Bos taurus taurus. This aspect has strongly contributed for the success of the embryo industry in Brazil. Little information is available to explain this difference. Our objective was to test the hypothesis that the difference in oocyte yield is a result of higher numbers of preantral follicles in the ovaries of B. indicus females. Ovaries (n = 44) of fetuses aging from 180 to 240 days and of heifers from 20 and 24 months were collected from Bos taurus indicus (Nelore) and Bos taurus taurus (Aberdeen Angus) females at a slaughterhouse. Fetal age was estimated from the crown-rump length (Evans HE and Sack WO 1973 Anat. Hist. Embryol. 2, 11-45). Ovaries from Nelore fetuses (n = 10) and heifers (n = 12) and Aberdeen Angus fetuses (n = 10) and heifers (n = 12) were cut longitudinally into two halves, fixed in Bouin’s solution, and processed for histological evaluation. The ovarian halves were dehydrated in alcohol, cleared with xylene, embbebed in paraffin, and serially sectioned at 7 μm. Every 120th section was mounted and stained with periodic acid Schiff and hematoxylin. The number of preantral follicles was estimated through the counting of follicles in each section using the nucleus of the oocyte as a marker and a correction factor (Gougeon A and Chainy GBN 1987 J. Reprod. Fertil. 81, 433-442). Only 1 ovary per female was analyzed. Preantral follicles were classified according to the developmental stage as primordial (1 layer of flattened granulosa cells surrounding the oocyte), primary (1 layer of cuboidal granulosa cells), or secondary (2 or more layers of cuboidal granulosa cells), and as normal or degenerated according to their morphological appearance. The number of preantral follicles was not normally distributed and was compared using the Mann-Whitney test. There was no difference (P > 0.05) between the average number of preantral follicles in the ovaries of Bos indicus and Bos taurus females. The average number of preantral follicles per female was 143 929 ± 253 (mean ± SD) and 285 155 ± 570 for Bos taurus and Bos indicus fetuses, respectively, and 76 851 ± 280 and 109 673 ± 293 for Bos taurus and Bos indicus heifers (P > 0.05), respectively. A large variation in numbers of preantral follicles was observed among individuals within the same category and between breeds. Our results suggest that there might be differences in mechanisms controlling follicle development after the preantral stage accounting for the greater oocyte yield from Bos indicus females. This work was supported by grants from the Universidad Nacional del Litoral (CAI+D program), Universidad Nacional de Lomas de Zamora, the Argentine National Agency for the Promotion of Science and Technology (ANPCyT), and the Argentine National Council for Science and Technology (CONICET).