1. The hypotheses tested were that foraging behavioural responses (number of meals consumed, the number of encounters with prey and feeding time) and activity (swimming time) of the freshwater predatory leech Nephelopsis obscura were higher in the dark than in the light, increased with the size of the predator and increased with the length of the starvation period prior to feeding. 2. While swimming activity was longer in the dark, not all the foraging behaviour variables followed this pattern. There was an inverse relationship between swimming time and the number of meals consumed and the poor correspondence between swimming time and number of prey encounters indicates that swimming is not always directly associated with increased foraging and might be related to movements between micro-habitats. 3. The effect of body size on feeding is complicated by its interaction with hunger levels and the ability of the predator to handle prey.