Protamines are abundant basic proteins involved in the condensation of sperm chromatin. In the mouse, protamine genes are transcribed postmeiotically in round spermatids. We have cloned and sequenced the mouse protamine 1 gene. Ten lines of transgenic mice harboring marked protamine 1 sequences were generated by microinjection of fertilized eggs. Transcription of the transgene is restricted to round spermatids and in several cases exceeds that of the endogenous gene. The cis-acting sequences required for tissue-specific protamine expression reside on a 2.4-kilobase restriction fragment. Prospects for using transgenic mice to address fundamental questions of male germ-cell development are discussed.