Drift activity of stream invertebrates typically is greatest during the nighttime hours in running waters throughout the world. Such diel periodicity may be an adaptive response that minimizes exposure to visually hunting, drift—feeding fishes. I tested this risk—of—predation hypothesis by examining drift behavior of mayflies in a series of Andean mountain and piedmont streams that vary in the abundance of drift—feeding fishes. Drift was primarily nocturnal in piedmont streams with natural populations of visually hunting predators. In contrast, mayfly drift activity did not differ between day and night in mountain streams that historically lack drift—feeding fishes. However, in naturally fishless Andean streams containing introduced rainbow trout, nocturnal peaks in drift were observed for the mayfly Baetis, suggesting a rapid evolutionary change in behavior in response to an exotic predator. When drift periodicity was examined along a gradient of predation regimes, activity was found to be increasingly restricted to the nighttime hours as predation risk became more intense. Diel periodicity was observed even when fish were experimentally excluded, suggesting that nocturnal activity has evolved as a fixed behavioral response to predation, and is not a direct ecological consequence of diurnal feeding by fishes. These observations support the hypothesis that predation risk is important in determining the timing of prey drift behavior.