SignificanceSpermatozoa have a streamlined shape to swim through the female reproductive tract to fertilize oocytes. However, little is known about the process of elimination of spermatid cytoplasm. When we createdTsks-null mice, the knockout mice cannot generate two types of nuage, reticulated body (RB) and chromatoid body remnant (CR), and are sterile with abnormal elimination of spermatid cytoplasm. Due to the absence of RB and CR,Tsksknockout spermatozoa have excess residual cytoplasm because the cytoplasmic contents cannot be eliminated from spermatid cytoplasm. These results suggest that TSKS-derived nuage are essential for spermatozoa to be streamlined. This study contributes to our understanding of spermiation, genetic diagnosis of idiopathic male infertility, and treatment of patients with infertility.