We recently reported that the Gtsf1/Cue110 gene, a member of the evolutionarily conserved UPF0224 family, is expressed predominantly in male germ cells, and that the GTSF1/CUE110 protein is localized to the cytoplasm of these cells in the adult testis. Here, to analyze the roles of the Gtsf1/Cue110 gene in spermatogenesis, we produced Gtsf1/Cue110-null mice by gene targeting. The Gtsf1/Cue110-null mice grew normally and appeared healthy; however, the males were sterile due to massive apoptotic death of their germ cells after postnatal day 14. In contrast, the null females were fertile. Detailed analyses revealed that the Gtsf1/Cue110-null male meiocytes ceased meiotic progression before the zygotene stage. Thus, the Gtsf1/Cue110 gene is essential for spermatogenesis beyond the early meiotic phase. Furthermore, the loss of the Gtsf1/Cue110 gene caused increased transcription of the long interspersed nucleotide element (Line-1) and the intracisternal A-particle (IAP) retrotransposons, accompanied by demethylation of their promoter regions. These observations indicate that Gtsf1/Cue110 is required for spermatogenesis and involved in retrotransposon suppression in male germ cells.