The role of the proto-oncogene Kit expression during gonadal development, then in differentiated spermatogonia has been thoroughly established. The present study was designed to investigate the consequences of a partial defect in Kit gene expression on sperm fertilizing ability, using Kit haplodeficient mice (kitW-lacZ/+). Same inbred mice (kit+/+) were used as controls. Epididymal sperm characteristics and in vivo fertility were assessed, then in vitro-fertilization experiments were carried out for mice of both genotypes. Epididymal sperm count was drastically reduced, and sperm motility was also decreased in kitW-lacZ/+ compared with kit+/+ males. Both in vivo or in vitro fertility were greatly reduced in kitW-lacZ/+ compared with kit+/+ males. By contrast, the fertility of kitW-lacZ/+ females was apparently unaffected. Additionally, a higher number of spermatozoa with undetected acrosomal contents was revealed by fluorescein isothiocyanate-labelled Pisum sativum agglutinin acrosomal staining after epididymal sperm retrieval in kitW-lacZ/+ mice, whereas no difference was observed after induction of acrosomal reaction in mice of either genotype. Ultra-structural data confirmed the higher frequency of abnormal acrosome in spermatozoa of kitW-lacZ/+ mice. Thus, sperm production is impaired in Kit haplodeficient mice both on a quantitative and a qualitative basis. Finally, we show that one single copy of Kit gene is not sufficient to maintain genuine fertility in male mice.