Cytoskeletal alterations in both Sertoli cells and germ cells are important during many facets of mammalian spermatogenesis. Diaphanous-related formin proteins are known to control many aspects of actin-based cytoskeletal rearrangements, yet nothing is known regarding the expression of formins in the testis. Accordingly, here we present the first data describing mDia1 and mDia2 mRNA and protein expression in primary Sertoli cell isolates, established tissue culture cell lines often used as models for Sertoli cell analysis, and mixed populations of adult rat male germ cells. Furthermore, we have examined intact sections of rat testis. The results suggest strongly that mDia1 and mDia2 are indeed involved in the regulation of Sertoli cell and germ structure during mammalian spermatogenesis, and provide strong indications of the future directions for mechanistic studies.