Radioactive fallout due to the Fukushima reactor explosion in Japan was detected in environmental samples collected in France. The presence of (131)I in aerosols (200±6 μBq m(-3)) collected at the Pic du Midi observatory, located at 2877 m altitude in the French Pyrénées, indicated that the Japanese radioactive cloud reached France between 22 and 29 March, i.e. less than two weeks after the initial emissions, as suggested by a (137)Cs/(134)Cs ratio of 1.4. Cesium radioisotopes ((134)Cs and (137)Cs) were not detected in this sample but they were present in the aerosol sample collected the next week, i.e. between 29 March and 05 April (about 10 μBq m(-3)). We also report (131)I activities measured in grass (1.1-11 Bq kg(-1); fresh weight) and soil samples (0.4 Bq kg(-1)) collected in the Seine River basin between 30 March and 10 April. The (134)Cs from the damaged Fukushima power plant was also detected in grass collected in the Seine River basin between 31 March and 10 April (0.2-1.6 Bq kg(-1) fresh weight, with a (137)Cs/(134)Cs ratio close to 1, which is consistent with Fukushima radioactive release). Despite the installation of a network of nested stations to collect suspended matter in the upstream part of the Seine River basin, (131)I was only detected in suspended matter (4.5-60 Bq kg(-1)) collected at the most upstream stations between 30 March and 12 April. Neither (131)I nor (134)Cs has been detected in environmental samples since the end of April 2011, because of the rapid decay of (131)I and the very low activities of (134)Cs (about 400 times lower than after Chernobyl accident).
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