Importin proteins control access to the cell nucleus by mediating the nuclear transport of specific cargoes. We hypothesized that developmental regulation of gene expression may be partially effected by changes in the nuclear transport machinery complement, manifested as regulated expression of importin α family genes. We first clarified the identity of the five known mouse importin α genes relative to those for human and then determined their expression throughout postnatal rodent testis using PCR and in situ hybridization. Distinct expression patterns were observed for each. At 10 dpp, all importin α mRNAs were detected in spermatogonia. In the adult mouse testis, importinsα1 and α3 were detected in spermatogonia and early pachytene spermatocytes. Importin α4 mRNA was identified in pachytene spermatocytes, α6 mRNA in round spermatids, and α2 mRNA in both of these. The distinct importin α expression patterns are consistent with their having specific roles and transport cargoes during spermatogenesis. Developmental Dynamics 235:253–262, 2006. © 2005 Wiley-Liss, Inc.