Detection of predators, social monitoring, and avoidance of infanticide have been invoked to explain patterns of vigilance. To test these hypotheses, I examined the vigilance of 120 Utah prairie dogs (Cynomys parvidens) using “one–zero” sampling on individuals of known sex, reproductive state, and genealogy. Male and female prairie dogs increased vigilance during periods of heightened predation risk (i.e., with few surrounding conspecifics) unless the risk resulted from spatial positioning in the colony. Reproductive males and females were more vigilant than their nonreproductive counterparts during breeding, and estrous females increased vigilance in the presence of prospective mates. In contrast to predictions of the infanticide-avoidance hypothesis, lactating females were less vigilant than females without litters when neonates were in a nursery burrow and vulnerable to infanticide by males. Males were more vigilant than females, but not when the offspring of their sexual partners were vulnerable to infanticide or opportunities to kill offspring increased. I conclude that vigilance in Utah prairie dogs varies mostly in relation to the risk of predation. Mate competition also increases vigilance in males and females. However, prevention or facilitation of infanticide does not appear to influence vigilance in males or females.