Late generations of telomerase-null (TR-/-) mice exhibit progressive defects in highly proliferative tissues and organs and decreased fertility, ultimately leading to sterility. To determine effects of telomerase deficiency on germ cells, weinvestigated the cleavage and preimplantation development of embryos derived from both in vivo and in vitro fertilization of TR-/- or wild-type (TR+/+) sperm with either TR-/- or TR+/+ oocytes. Consistently, fertilization of TR-/- oocytes with either TR+/+ or TR-/- sperm, and TR-/- sperm with TR+/+ oocytes, resulted in aberrant cleavage and development, in contrast to the normal cleavage and development of TR+/+ oocytes fertilized by TR+/+ sperm. Many (>50%) of the fertilized TR-/- eggs developed only one pronucleus, coincident with increased incidence of cytofragmentation, in contrast to the normal formation of two pronuclei and equal cleavage of wild-type embryos. These results suggest that both TR-/- sperm and oocytes contribute to defective fertilization and cleavage. We further found that a subset (7-9%) of telomeres was undetectable at the ends of some metaphase I chromosomes from TR-/- spermatocytes and oocytes, indicating that meiotic germ cells lacking telomerase ultimately resulted in telomere shortening and loss. Dysfunction of meiotic telomeres may contribute to aberrant fertilization of gametes and lead to abnormal cleavage of embryos, implying an important role of functional telomeres for germ cells undergoing fertilization and early cleavage development.