Abstract. Scan rates of eight species of common ungulate prey of the African lion, Panthera leo, were measured in Serengeti National Park, Tanzania, to examine the effect of predation risk on vigilance. Vigilance was measured both during lion hunts and when lions were not present. Risk of predation was expected to vary with herd size, prey species and density, available light and cover, predator presence and predator activity. Risk was expected to decline in large herds due to dilution. Over all prey species, vigilance declined with increasing group size for three measures of herd size: (1) total herd size, (2) numbers of conspecifics and (3) number of heterospecifics in a herd. Species that were at high predation risk (measured as hunts recorded per herd seen) had low vigilance. Risk was expected to be high near dense cover and when predators were most active. Wildebeests, Connochaetes taurinus , and zebras, Equus burchelli, scanned more when lions were most active. However, wart hogs, Phacocheroerus aithiopicus, and wildebeests scanned less where more cover was available for lions. These results suggest that scan rates vary not only with predation risk, but also with the utility of scanning. Lions did not appear to adjust their stalking behaviour according to the vigilance of the prey group.