Plutonium-239 (Pu) and plutonium-240 (Pu) activity concentrations and Pu/Pu atom ratios are reported for Brown Algae (Fucus distichus) collected from the littoral zone of Amchitka Island (Alaska), and at a control site at Unalaska, Alaska. The average Pu/Pu atom ratio observed in dried F. distichus collected from Amchitka Island was 0.227 ± 0.007 (N = 5) and compares with the expected Pu/Pu atom ratio in integrated worldwide fallout deposition in the Northern Hemisphere of 0.1805 ± 0.0057. In the absence of any evidence of a local source of plutonium containing an elevated Pu/Pu isotopic signature, the characteristically high Pu/Pu content of F. distichus supports the view of the existence of a discernible, basin-wide non-fallout source of plutonium entering the subarctic Pacific.