Ecologically, spiders are both predators and prey. Therefore, they must balance being aggressive enough to forage successfully, but not so aggressive that they become overly exposed to predation. Some species of spiders actively forage during clearly defined periods of the day, leading to the hypothesis that they should be less aggressive (or more defensive) during periods when they are not foraging, predicting that antipredator behaviour should be more pronounced during inactive foraging times. We tested the antipredator 'huddle response' in a nocturnal foraging orb-weaver, Larinioides cornutus, and found that, as predicted, the spiders huddled longer in the day than at night. We then conducted tests to determine whether the cycling of the response was regulated by an internal clock (circadian), and we found that huddle duration continued to cycle under constant dark (with periodicity significantly less than 24. h) as well as under constant light (periodicity nonsignificantly longer than 24. h). This work adds a novel behaviour to the list of behaviours under circadian control and also to the surprisingly short list of studies demonstrating circadian rhythm in spiders.