Glacial retreat during the Pleistocene revealed newly-usable habitats, allowing species colonization via founder events. This colonization is thought to be responsible for an evident loss in haplotype and nucleotide diversities in some terrestrial and estuarine species with increasing latitude. With the mid-shelf zone (30–200m) near shore along the Pacific coast of North America, due to a narrow and steep continental shelf, I looked to see if the loss of genetic diversity with increasing latitude extended to a middle-shelf flatfish, Pacific sanddab (Citharichthys sordidus). The mitochondrial DNA control region sequence from 130 Pacific sanddabs, taken from 4 locations, was compared utilizing all sequence differences and the transvertions only, due to high haplotype diversity. Overall significantly more differences, hence more diversity, were in the mismatch distribution in the lower latitudes. Additionally, compared to Dover sole, Pacific sanddab is more diverse, possibly due to older populations or less fishing pressure.