A transgenic mouse line expressing the CreERTM fusion protein under the control of the Math1 enhancer was generated. Expression of the transgene in the postnatal mouse was restricted to hair cells of the inner ear and granule neurons in the external granule layer of the cerebellum in a temporally regulated manner. Cre activity was virtually nonexistent in uninduced mice; however, treatment of newborn pups with tamoxifen, leading to nuclear translocation of the fusion protein, resulted in efficient recombination at LoxP sites in the appropriate cell types. Up to two thirds of cerebellar granule neurons and 80–90% of cochlear hair cells underwent Cre-specific recombination. This mouse line provides a powerful tool to dissect gene function at early and late stages in development of the cerebellum and inner ear. Developmental Dynamics 235:2991–2998, 2006. © 2006 Wiley-Liss, Inc.