The seminal vesicle is a male accessory sex organ that develops from segments of the Wolffian duct adjacent to the urogenital sinus. It produces most of the seminal plasma in both humans and rodents. To date, very few transcription factors have been linked to the development and differentiation of seminal vesicles. In this study, we have examined the role of basic helix-loop-helix (BHLH) B8 transcription factor expressed at high levels in the adult seminal vesicle and during seminal gland differentiation. Immunofluorescent studies indicate that BHLHB8 is expressed within the epithelial layer of the seminal layer of the seminal vesicle following branching morphogenesis but prior to full maturation of cell morphology and function. Analysis of mice that do not express BHLHB8 (Bhlhb8(-/-)) indicates no deficiency in the initial development of the seminal vesicle. However, morphological and ultrastructural analysis indicates disruption of the epithelial cellular architecture. The seminal vesicle epithelial layer of 2-mo-old Bhlhb8(-/-) mice shows extensive cellular degeneration based on the appearance of reduced microvilli, altered granule size, and dilated endoplasmic reticulum and Golgi apparatus. The seminal vesicle epithelial cells also degenerate prematurely, as evidenced by disruption of nuclear architecture and significant accumulations of autophagic bodies. These results identify BHLHB8 as a regulator in establishing and stabilizing the secreting epithelial cells of the seminal vesicle.