On a genomic level most SNPs show very small and non-significant differentiation among populations of Atlantic cod as measured by F_ST. Yet other SNPs are outliers in F_ST analysis  showing a range of F_ST values even approaching fixation of alternative alleles. This pattern is most likely due to natural selection. I am studying details of nucleotide variation at such outlier loci to understand the nature of selection. In this lecture I shall report on nucleotide variation and patterns of selection at a CKM locus showing extreme differentiation.