Intraspecific variation in nuclear DNA content (genome size) was measured for the Arctic ciscoCoregonus autumnalisby fluorescence flow cytometry with the fluorochrome 4′,6-diamidino-2-phenylindole (DAPI). Frozen erythrocytes from the domestic chickenGallus domesticuswere analyzed simultaneously with Arctic cisco samples, providing an internal reference standard. Mean DNA content for 140 Arctic ciscoes from seven sites along the Beaufort Sea coast of Alaska and Canada was 5.99 pg/cell. Sample variances and mean DNA content were significantly heterogeneous among coastal sites. A significant increasing linear trend in mean DNA content was found on a west-to-east cline along the Beaufort Sea coast. Intraspecific variation in genome size observed in this study was substantial, which indicates that genome size characterizations of species based on small samples from a single population may be grossly inadequate.