Heat-shock protein 110 (HSP110) family members act as nucleotide exchange factors (NEF) of mammalian and yeast HSP70 chaperones during the ATP hydrolysis cycle. In this study, we describe the expression pattern of murine HSPA4, a member of the HSP110 family, during testis development and the consequence of HSPA4 deficiency on male fertility. HSPA4 is ubiquitously expressed in all the examined tissues. During prenatal and postnatal development of gonad, HSPA4 is expressed in both somatic and germ cells; however, expression was much higher in germ cells of prenatal gonads. Analyses ofHspa4-deficient mice revealed that all homozygous mice on the hybrid C57BL/6J×129/Sv genetic background were apparently healthy. Although HSPA4 is expressed as early as E13.5 in male gonad, a lack of histological differences betweenHspa4−/−and control littermates suggests thatHspa4deficiency does not impair the gonocytes or their development to spermatogonia. Remarkably, an increased number of theHspa4-deficient males displayed impaired fertility, whereas females were fertile. The total number of spermatozoa and their motility were drastically reduced in infertileHspa4-deficient mice compared with wild-type littermates. The majority of pachytene spermatocytes in the juvenileHspa4−/−mice failed to complete the first meiotic prophase and became apoptotic. Furthermore, down-regulation of transcription levels of genes known to be expressed in spermatocytes at late stages of prophase I and post-meiotic spermatids leads to suggest that the development of most spermatogenic cells is arrested at late stages of meiotic prophase I. These results provide evidence that HSPA4 is required for normal spermatogenesis.