BACKGROUND: This study aims to investigate the anatomical differences in the anterior segment of the eyes between dogs and cats using ultrasound biomicroscopy (UBM) to understand the higher prevalence of primary angle-closure glaucoma (PACG) in dogs compared to cats. Retrospective analysis was performed on clinical data from 16 eyes of 16 dogs and 14 eyes of 14 cats with normal eye conditions. UBM was utilized to measure nine specific parameters, including Schwalbe's Line Distance (SLD), Iridocorneal Angle (ICA), Angle-Opening Distance (AOD), and three ciliary cleft parameters: width (CCW), length (CCL), and area (CCA). To account for differences in body size, ciliary cleft parameters were adjusted accordingly. RESULTS: Significant anatomical differences in the anterior segment were found between the two species. Dogs had smaller values for SLD, ICA, AOD, and ciliary cleft parameters (CCW, CCL, CCA) compared to cats. Even after body-size adjustment, the rectified ciliary cleft parameters remained smaller in dogs. CONCLUSION: The anatomical differences, particularly the smaller ciliary cleft and narrower drainage angles in dogs, may contribute to the higher prevalence of PACG in this species. Conversely, the larger ciliary cleft in cats may explain the lower occurrence of primary glaucoma in cats.