The aquatic isopod Asellus aquaticus can in some lakes be found as two different ecotypes; one in the habitat dominated by reed and one in stonewort stands. These ecotypes have been shown to differ in size, color and behavior. The reed ecotype is larger, darker and more active compared to the stonewort ecotype. In the two habitats there are different dominating predators: Invertebrate predators in the reed habitat and fish in the stonewort habitat. This project aims to examine how the presence of invertebrate predators and fish affect the two ecotypes of the isopod in regard to behavior and size composition in different substrates. To examine the effect on behavior the activity of isopods collected from Lake Tåkern was measured without and with chemical cues from perch and damselfly larva. The result of the behavior experiment showed no difference between the ecotypes nor the treatments. The lack of differences in the behavior indicates that there could be a variation between lakes. The effect on the size composition was examined by subjecting groups of isopods to predation by perch or damselfly larva in different substrate. The mean length of the group was measured before and after the trials. The size decreased significantly for the reed ecotype in stonewort substrate when subjected to predation by perch. The size for the stonewort ecotype increased significantly in reed substrate with damselfly larva as predator. The effect on size supports that the predators are the cause of the size difference between the ecotypes.