The beachflea [Orchestia gammarellus (Pallas)] is a trans-Atlantic amphipod inhabiting the littoral fringe. In Iceland, its distribution is temperature-limited; it has recently colonized Iceland's relatively warm south-western coast, and in the cooler north-west, several small populations inhabit isolated warm springs. We address two questions: (i) Do the warm spring populations show evidence of long-term residence in Iceland, or of recent colonization? (ii) For the new south-western Icelandic populations, can the source population be determined? We sequenced COI for 22 populations in Iceland, Nova Scotia, the Faroe Islands, the British Isles, Norway and Sweden. Morphometric analysis of a subset of populations assessed 16 continuous and five discrete characters. Genetically, we found a star phylogeny: a common haplotype was found at all sites except two neighbouring warm spring populations, and all haplotypes were within two base pairs of this common haplotype. Morphometrically, almost all populations examined differed significantly in some characters; however, the warm spring populations differed slightly more from each other than did other populations. Although the origins of the Icelandic populations could not be well resolved, our data are consistent with a recent European origin.