Information about past periods of climate change can provide a framework for investigating how marine communities may have adapted to changes both geographically and ecologically. The spatial distribution of variable haplotypes from the nuclear ribosomal DNA internal transcribed spacer (ITS15.8S-ITS2) region and a ribosomal DNA group-I intron from the obligate, rocky shore intertidal red alga Porphyra umbilicalis were used to reconstruct its biogeographical history since the last glacial maximum in the North Atlantic. Haplotype distributions from European and North American samples representing the range of P. umbilicalis are consistent with the hypothesis that North American populations were extirpated during the last glacial maximum and subsequently recolonized from European donor populations. A non-coding intergenic region between the mitochondrial cytochrome oxidase (cox)2 and cox3 genes was also investigated, but because there were extremely low levels of intraspecific variation, the spacer was not useful for phylogeographical analysis.