SUMMARY Some factors influencing the causation of stillbirths in cattle were studied in 105,122 progeny of 100 bulls of eight breeds over a period of 8 years. The breed of dam was not considered. The calves of Friesian bulls were studied in greater detail than those of seven other breeds, in view of the greater numbers available. Significant differences in the rates of stillbirth were found between the sires of calves within the Friesian, Guernsey and Charollais breeds. A related line of Friesian bulls produced a higher incidence of stillbirths than the remaining group of unrelated bulls. There were also differences in the rates of stillbirth between the eight breeds of sires studied. These findings suggest a genetic influence on this trait. An observed frequency distribution of all the sires grouped in classes of 1 per cent based on the values of the rates of stillbirth of their calves showed them to be normally distributed. This distribution permits selection for this trait. There were significant differences in the rates of stillbirth during different years in the Friesian, Guernsey and combined breeds. The pattern of variation of the annual incidence of stillbirths in the Friesian closely resembled that of the combined breeds. The trend in the Guernsey was different. No simple explanation was possible as annual effects were likely to be confounded with many environmental factors. Male calves appeared more liable to be stillborn and the incidence of stillbirths among them was higher than in the female calves. There were no differences in the rates of stillbirth to first inseminations by fresh or frozen semen. The rates of stillbirths were found to increase with higher order numbers of insemination, possible because the dams provided an unfavourable maternal environment. None of the correlations of the rate of stillbirth and other factors studied in bulls was statistically significant. It is hoped that consideration will be given to this character in the future choice of a selection-index for the evaluation of bulls.