A study was conducted to determine the 137 Cs activity concentrations in various seaweed and mussel samples from the North East Mediterranean Sea, including the brown algae (Dictyota dichotoma) and oarweed (Laminaria digitata), and mussel species (Haliotis asinina, Mytilus edulis and Mytilus galloprovinciali). The 137 Cs activity concentration in seaweed species exhibited varying levels, ranging from 84.20 ± 1.82 to 236.05 ± 5.72 mBq kg -1 fresh weight (fw) for oarweed, and 106.29 ± 2.26 to 252.38 ± 6.04 mBq kg -1 fw for brown algae. The 137 Cs value in mussel species were between 12.94 ± 0.51-101.84 ± 1.35 mBq kg -1 fw for M. galloprovincialis, 15.49 ± 0.58-71.52 ± 1.16 mBq kg -1 fw for M. edulis, and 10.36 ± 0.11-69.13 ± 1.34 mBq kg -1 fw for H. asinine. When comparing the 137 Cs concentration in seaweed and mussel species, the average concentration in seaweeds was approximately four times higher than that in mussels. This significant difference in radioisotope concentration highlights the potential for higher bioaccumulation of 137 Cs in seaweeds compared to mussels., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)