Animals exhibit diel periodicity in their activity in part to meet energy requirements whilst evading predation. A competing hypothesis suggests that partitioning of diel activities is less important because animals capitalise on opportunity. To test these hypotheses we examined the diel activity patterns for two cyprinid minnows, chubbyhead barb Barbus anoplus and the Eastern Cape redfin minnow Pseudobarbus afer that both occur within headwater streams in the Eastern Cape, South Africa. Chubbyhead barbs exhibited consistent nocturnal activity based on both field and laboratory observations. Due to the absence of fish predators within its habitat, its nocturnal behaviour suggests a response to the cost associated with diurnal activity, such as predation risk by diving and wading birds. In contrast, redfin minnows showed high diurnal activity and a shoaling behaviour in the wild, whereas, in the laboratory, they showed high refuge use during the diel cycle. Despite their preference for refuge in the laboratory, they were diurnally active, a behaviour that was consistent with observations in the wild. The diurnal activity of this species suggests a response to the cost associated with nocturnal activity. Such a cost could be inferred from the presence of the longfin eel, a native predator that was active at night, whereas the daytime shoaling behaviour suggests an anti-predator mechanism to diurnal visual predators. The implications of these findings relate to the impacts associated with the potential invasions by non-native piscivores that occur in the mainstem sections. Diurnal activity patterns for redfin minnows, that are IUCN-listed as endangered, may, in part, explain their susceptibility to high predation by visual non-native piscivores, such as bass and trout. In contrast, the nocturnal habits of chubbyhead barbs suggest a probable pre-adaptation to visual predation. The likelihood of invasion by nocturnally-active sharptooth catfish Clarias gariepinus, however, may compromise this prior advantage.