Alaria (Alariaceae, Phaeophyceae) is a common genus of kelps in the northern hemisphere. Fourteen species are currently recognized, of which three, Alaria esculenta (L.) Greville, A. pylaii (Bory de Saint-Vincent) Greville and A. grandifolia J. Agardh, are reported for the cold-temperate North Atlantic Ocean. Alaria esculenta, the type species described originally from the North Atlantic, exhibits a range of biogeographically correlated morphotypes suggesting the possibility of multiple species, subspecies or hybrids. In Ireland we discovered an A. esculenta population with unusually long stipes resembling the type specimen of A. grandifolia described from Spitsbergen by J. Agardh in 1872. These and other plants of A. esculenta from Ireland were compared with plants from Spitsbergen fitting the description of A. grandifolia, using sexual hybridization, relative growth rate measurements and DNA sequence comparisons. Complete interfertility was observed between the different isolates. Three nucleotide substitutions (0·37 %) were found in the rbcL and RuBisCo spacer of A. grandifolia, and two in the partial 18S rRNA gene and ITS1 sequences. The relative growth rate at 10 °C of an Irish self-cross was significantly lower than those of all the other crosses. Comparison of RuBisCo spacer sequences of the Spitsbergen A. grandifolia and six A. esculenta isolates showed that A. grandifolia was identical to A. esculenta from Halifax, Canada. The partial 18S rRNA gene and ITS1 sequence of A. grandifolia was identical to that of A. praelonga from Japan and differed by a single substitution from A. esculenta from Scotland and by two nucleotide substitutions from the isolate from Ireland. The intraspecific differences in A. esculenta, together with the hybridization and morphometric results, suggest that A. grandifolia is to be considered conspecific with A. esculenta, and that A. grandifolia is most probably a large deep-water morphological variant, subspecies or ecotype of A. esculenta.