Studies of the daily activity patterns of Octopus vulgaris have shown varying activity in different habitats. This might stem from the octopus' ability to respond to influences such as predation pressure by adjusting its activity pattern. To test the hypothesis that a predatory threat could alter activity cycles, six octopuses were each held in a partitioned tank with a potential predator for a week. After we had determined the circadian activity of each undisturbed subject for two days, a nocturnal (moray eel, Muraena helena) and a diurnal fish (triggerfish, Balistes carolinensis) were alternately introduced into the second compartment of the tank for seven days. Each of these periods was then followed by a four-day period without eel or triggerfish.