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Abstract: The distribution of technetium-99 (99Tc) in annual growth segments of the brown seaweed Ascophyllum nodosum (Fucales, Phaeophyceae) from the southwestern coast of Norway is examined in samples collected from January to November 2006. A twenty-fold increase in the 99Tc-concentration from the youngest to the oldest growth segments was found. The concentrations ranged from 42 to 98Bq/kg dry weight (d.w.) and from 964 to 1000Bq/kg d.w. in growth segments formed in 2006 and 1996, respectively. In addition, a seasonal variation in the 99Tc concentration was observed in the actively growing 2006-segments: concentrations decreased from 98Bq/kg d.w. in April to 54Bq/kg d.w. in June; there was a further reduction from June to August (42Bq/kg d.w.); and, finally there was an increase from August to November (93Bq/kg d.w.). In most of the segments formed between 2000 and 2005, there was a tendency of slightly decreasing 99Tc-concentrations between June and November but this pattern was not observed for the older growth segments. In order to find an explanation for the non-homogenous distribution of 99Tc within thalli of A. nodosum, different hypotheses are discussed. Uptake and elimination of 99Tc appears to be most pronounced in the actively growing segments. To date, such non-homogenous distribution of 99Tc within thalli of A. nodosum has not been taken into consideration, neither in connection with sample collection nor analysis. This paper shows that special protocols must be followed if A. nodosum is going to be used as a bioindicator for 99Tc in the marine environment. A sampling strategy is proposed. [Copyright &y& Elsevier]