Using an anti-Fos family member antiserum, we previously described, in the testis of Rana esculenta, the presence of a nuclear 43-kDa protein that we hypothesized to be Fra-1. Using an antiserum against Fra-1, we here report on Fra-1 expression, localization, and putative activity in the R. esculenta testis during the annual reproductive cycle. Western blot analysis confirms that the nuclear 43-kDa protein is Fra-1. Immunocytochemistry demonstrates Fra-1 in peritubular myoid cells (PMC), efferent ducts, and blood vessels. We present, for the first time for a vertebrate, experimental evidence that the expression of Fra-1 in PMC is related to its activity during sperm transport from the tubular compartment to the efferent ducts.