At the beginning of the breeding season an eight-year-old standardbred stallion had semen with virtually zero sperm motility and an approximately 90 per cent incidence of midpiece and tail defects. The motility of the sperm improved to 7 per cent when semen was collected daily but its morphology did not improve. Electron microscopy revealed that the defects consisted mainly of a loss of microtubules in the axoneme and of disorganised midpieces. A pregnancy rate of 24 per cent per cycle and 44 per cent for the season was achieved in 32 mares after the insemination of whole ejaculates collected from the stallion frequently. The fertility was much higher than would have been expected from the characteristics of the semen. It is concluded that this sperm defect, reminiscent of the 'Dag defect' in bulls and the defect in T-locus mice, does not render the animal infertile.