A mouse was generated containing a floxed exon 3 of the gene for the kinesin family member KIF9. By in situ hybridization, expression of KIF9 mRNA was highest in the testis and was also strong in epithelia containing multi-ciliated cells such as the ependyma, bronchioles and oviduct. Deletion of the exon led to loss of KIF9 expression at the mRNA and protein level with no effect on viability. However, homozygous KIF9 knockout males were sterile. Although KIF9 knockout sperm were motile, they were unable to fertilize oocytes in an in vitro fertilization assay. Closer examination of sperm motility indicated a subtle difference in waveform. Our results suggest that KIF9 plays a role male fertility, possibly through regulation of flagellar waveforms in ciliated cells.