Summary Ovarian follicular activity was studied by ultrasonography during 17 oestrous cycles in 9 Mangalarga mares during the second half of the ovulatory season. Sixteen oestrous cycles were considered normal and one 3‐wave cycle showing a prolonged luteal phase was considered atypical. Daily ultrasonographic examinations were performed and the compiled data on follicular dynamics were studied retrospectively. One major wave of follicular growth was observed in 13 of the 16 normal cycles (81.25%), whereas 2 major waves occurred in 3 cycles (18.75%). The mean (± s.d.) days of emergence of the primary wave of follicular development in cycles containing one or 2 waves were Day 6.0 ± 2.3 and Day 11.0 ± 1.0, respectively. The secondary wave of follicular development in 2‐wave cycles emerged on Day 0.0 ± 3.6. The day of wave divergence for primary waves of follicular development in cycles which exhibited one or 2 major waves were Day 12.2 ± 3.5 and Day 173 ± 3.0, respectively. Divergence of secondary waves occurred in only one of the 3 cycles which exhibited 2 major follicular waves (Day 7). The mean (± s.d.) maximum diameters of the dominant follicle in the primary wave of oestrous cycles exhibiting one and 2 major waves were 39.0 ± 3.9 mm and 34.7 ± 2.5 mm, respectively. The mean (± s.d.) maximum diameter of the dominant follicle present in the secondary wave was 34.3 ± 11.0 mm. The mean (± s.d.) lengths of the interovulatory intervals for cycles containing one and 2 major waves were 19.4 ± 2.2 and 23.3 ± 2.5 days, respectively. These data indicate that most Mangalarga mares show one major follicular wave during the oestrous cycle but a small percentage of mares show 2 major waves.