Four different swimming behaviours of the backswimmer, Anisops deanei, each characterized by a different angular position of the metathoracic legs and speed, are described along with the effect of water temperature and density on (1) the number of each behaviour exhibited and (2) total time spent swimming, per unit time. Generally, as density increased the swimming activity in each behaviour decreased while as temperature increased swimming activity increased. The direction of the effects were the same when total time spent swimming per unit time was examined. Nearest neighbour analysis showed that A. deanei form obvious aggregations in laboratory tanks as density increases, and this effect remains significant at least to the third neighbour. Density had a significant effect on neighbour distances whereas temperature did not. It was hypothesized that the primary function of these aggregations, which are also commonly observed in the field, is an anti-predator behaviour. Laboratory experiments using models of a common sit-and-wait predator of A. deanei, showed that although the encounter rate increased with both water temperature and density the observed encounter rate at the higher densities was significantly less than expected with a random distribution of prey. It is suggested that this defence effect would also exist with more mobile predators like fish.