Ionizing radiation can induce various cellular responses, including DNA damage that may lead to mutations or cell death [1,2]. The micronucleus assay in Allium cepa cells is a well-established method for evaluating genotoxic effects and establishing dose-response relationships [1,3,4]. In this study, we investigated how different β-radiation dose rates affect the frequency of radiation-induced micronuclei and the Mitotic Index in Allium cepa meristematic cells. Meristematic cells of Allium cepa were irradiated using β-particles emitted from ⁹⁰Sr+⁹⁰Y sources, with varying dose rates. The micronucleus assay was employed to quantify the frequency of micronuclei, which are indicative of chromosome fragments or whole chromosomes that are not incorporated into the daughter nuclei during cell division. Additionally, we assessed the Mitotic Index to determine the proportion of actively dividing cells, providing insights into cellular proliferation and potential radiation effects. Our findings underscore the significant influence of β-radiation dose rate on micronuclei formation in Allium cepa cells. Higher dose rates correlated with increased micronuclei frequency, indicating greater genotoxic stress. This observation highlights the importance of considering dose rate variations in radiation dosimetry and risk assessment scenarios. Moreover, we emphasize the complementary role of the Mitotic Index in cytogenetic assessments. While micronucleus frequency reflects genomic instability, changes in the Mitotic Index provide context regarding cellular proliferation and the overall health of the meristematic tissue. Therefore, integrating both endpoints enhances the accuracy and reliability of genotoxicity assessments in radiation biology studies using Allium cepa as a model organism. This study contributes to the broader understanding of radiation-induced cytogenetic endpoints and their implications for environmental and occupational radiation protection [5,6,7]. By elucidating the dose-rate-dependent effects on micronucleus frequency and Mitotic Index in Allium cepa, our research supports informed decision-making in radiation safety regulations and practices. The authors acknowledge the Brazilian agencies CNPq (grants 307621/2023-5, 443464/2023-5) and CAPES (grant 88887821981/2023-00) for partial financial support.