Relatively large amounts of radioactive iodine I-129 (T (1/2) = 15.7 Ma) have been documented in seawater such as the English Channel, the Irish Sea and the North Sea. Data on the concentration of the iodine isotopes in waters of the Celtic Sea are missing. Aiming to provide first I-129 data in the Celtic Sea and compare them with levels in the other close-by seawater bodies, surface seawater samples were analyzed for the determination of I-127 and I-129 concentrations. The results revealed a high level of I-129 in these waters and suggest strong influence by liquid discharges from La Hague and Sellafield reprocessing facilities. I-127 concentrations are rather constant while the I-129/I-127 ratio reaches up to 2.8 x 10(-8) (ranging from 10(-10) to 10(-8)), which is 2-4 orders of magnitude higher than pre-nuclear era natural level. Transport of I-129 to the Celtic Sea is difficult to depict accurately since available data are sparse. Most likely, however, that discharges originated from La Hague may have more influence on the Celtic Sea I-129 concentrations than the Sellafield. Comprehensive surface water and depth profiles I-129 data will be needed in the future for assessment of environmental impact in the region.