Catharanthus roseus is an important medicinal plant containing alkaloids such as vinblastine and vincristine, which have strong cell division inhibitory activity and can serve as a colchicine alternative in polyploidization programs. This study aimed to evaluate the effects of aqueous and ethanolic leaf extracts of C. roseus on cell division in meristematic cells of onion (Allium cepa) roots. Extracts at four concentrations (0, 20, 50, and 100%) were applied to onion bulbs. The experiment was conducted as a completely randomized factorial design with three replications. Traits assessed included root length and number, rooting time, mitotic and phase indices, chromosome counts, and cytological abnormalities such as multinucleation and chromatid bridges. Results showed that increasing extract concentration reduced root length, and rooting was delayed in the ethanolic extract, with the greatest delay observed at 100% concentration. The highest prophase index was recorded at 20% extract for both types, while the highest telophase index occurred at 100% ethanolic extract. Mitotic index peaked at 20% and 50% ethanolic extracts. Chromosomal abnormalities, including stickiness, breakage, and duplication, were mainly observed in 50% ethanolic extract, whereas chromatid bridges appeared only in 50% aqueous extract. These findings indicate that C. roseus alkaloids can influence mitosis and may be applied in breeding programs and polyploidization of medicinal plants.