ABSTRACT: Extrinsic factors such as the physical environment can control the activities of animals, but their effect on behaviour can vary substantially among individuals within a given population depending on more intrinsic factors such as differences in the need to search for food or mates. We examined the activity of the brooding intertidal gastropod Littorina saxatilis under a range of different physical stressors experienced during high tide to test 2 hypotheses: (1) activity is affected by hydrodynamic conditions and (2) different reproductive groups respond in different ways to these conditions, specifically that gravid females exhibit more risk-avoidance behaviour in relation to other individuals. Two components of activity (magnitude of movement and shelter use) were measured for 3 reproductive groups (males, non-gravid females and gravid females) under temporally varying conditions of different environmental factors, including water flow, maximum wave force and water temperature. Movement (specifically, net distance displaced) of snails decreased with increases in water flow and maximum wave force. Reproductive groups did not differ in the form of their response function to water flow, but instead differed in the magnitude of their response, with gravid females moving the shortest distances. Gravid females were also the only group to increase their use of shelters in response to increases in maximum wave force, and this response was positively related to their fecundity. We interpret these behaviours as responses to increased risk of dislodgment and conclude that the behaviour of gravid females (i.e. use of shelters and reduced movements) should increase their survival as well as that of the offspring they carry.