The genome sizes of 8 species of amphipods collected from the Canadian Arctic were estimated by flow cytometry. Haploid genome sizes ranged from 2.94 +/- 0.04 pg DNA in Acanthostepheia malmgreni (Oedicerotidae) to 64.62 +/- 2.85 pg in Ampelisca macrocephala (Ampeliscidae). The value for Ampelisca macrocephala represents the largest crustacean genome size recorded to date (and also the largest within the Arthropoda) and indicates a 400-fold variation in genome size among crustaceans. The presence of such large genomes within a relatively small sample of Arctic amphipods is striking and highlights the need to further explore the relationships between genome size, development rates, and body size in both Arctic and temperate amphipods.