Our goals were to compare the transcriptomes of two closely related species of killifish, the rainwater and bluefin killifish. These two species are interesting because one is a freshwater species and the other is euryhaline meaning that it can tolerate both fresh and saltwater. We compared the areas of the genome where these two species differ to determine if the areas involved in species differences are also involved in osmoregulation. We also compared populations within species - particularly in the rainwater killifish. This species has permanent freshwater and saltwater populations. We asked whether these populations also showed pronounced divergence in ion/osmoregulatory loci.