Translational control is a major form of regulating gene expression during gametogenesis and early development in many organisms. We sought to determine whether the translational repression of the protamine 1 (Prm1) mRNA is necessary for normal spermatid differentiation in mice. To accomplish this we generated transgenic animals that carry a Prm1 transgene lacking its normal 3' untranslated region. Premature translation of Prm1 mRNA caused precocious condensation of spermatid nuclear DNA, abnormal head morphogenesis, and incomplete processing of Prm2 protein. Premature accumulation of Prm1 within syncytial spermatids in mice hemizygous for the transgene caused dominant male sterility, which in some cases was accompanied by a complete arrest in spermatid differentiation. These results demonstrate that correct temporal synthesis of Prm1 is necessary for the transition from nucleohistones to nucleoprotamines.