We previously identified a mutation in the DIAPH3 (Diaphanous homolog 3) gene in a family with a form of hereditary deafness known as AUNA1. Patients with this mutation develop hearing loss in the late teenage years, which rapidly progresses to profound deafness. Because it is difficult to directly study genes and proteins in the human inner ear, we created a mouse model with abnormal expression of Diap3, the mouse gene corresponding to the human DIAPH3 gene. Two lines of transgenic mice were generated, both with progressive deafness and normal outer hair cell function as measured by otoacoustic emissions, similar to humans with AUNA1 deafness. Examination of the inner ears in the deaf mice shows that the stereocilia of the inner hair cells appeared abnormally long and are fused together by age 6 months. Furthermore, there is a significant reduction in the number of inner hair cell ribbons in the cochlea. DIAPH3 belongs to a family of genes encoding diaphanous-related formins, which are important regulators of actin. These mutant mice will yield insights into the role of these proteins in the cochlea, which may involve coordinating actin in the maintenance and assembly of the stereocilia and ribbon synapse, and will be useful to test therapies to protect and restore hearing.