Three-spined stickleback is believed to be a key species in the food web of Norwegian freshwater Lakes with extensive linkages to the parasite community. They are often found infected with many different parasite species and are commonly preyed upon by piscivorous fish. In this project we study the importance of sticklebacks and how they are connected to the parasite community found in freshwater lakes in Norway. Firstly we will compare the parasite community of a recipient stickleback population with that of its nearby source population to study possible introductions of new parasites. Secondly, by studying the parasites of fish populations with different fish species compositions we aim to understand how the presence of stickleback affects the fish parasite community. Finally, with extensive parasitological data from Lake Takvatn, we aim to explain temporal changes in the parasite community of a fish community that has previously undergone heavy regulation.