Helminths are frequent parasites of natural fish populations but interactions of helminths with the fish immune system are under-studied. Infection rates often vary substantially among host populations, on local and global scales. In southcentral Alaska, oceanic stickleback populations have colonized freshwater habitats repeatedly and independently in the 12,000 yr since the last glacial maximum, resulting in a freshwater adaptive radiation. Upon colonization of freshwater, oceanic stickleback first encounter S. solidus, a trophically transmitted cestode that has a long evolutionary history of parasitizing freshwater stickleback but is not viable in marine environments. Initial work in British Columbia suggests, as expected, that oceanic stickleback exhibit little resistance to infection, whereas freshwater populations exhibit lower infection rates under experimental conditions, suggesting evolved resistance. A crucial step in understanding the interaction in such systems is to examine whether stickleback populations exhibit underlying differences in immune expression profiles. To do so, I am looking at immune profiles of different Alaskan freshwater populations as well as one oceanic population as ancestral proxy, which have been raised under controlled conditions.