ABSTRACT. – Radionuclide experiments were performed to study the accumulation, retention and transport of ²⁴¹Am and ¹³⁴Cs in eggs of the common cuttlefish Sepia officinalis. Experiments were designed to determine the processes controlling uptake of both radionuclides following exposure via seawater. The bioaccumulation of both radionuclides displayed saturation kinetics with steady state in uptake being reached in 10 d for ²⁴¹Am and 6 d for ¹³⁴Cs. ²⁴¹Am was readily taken up by the eggs which reached a concentration factor (CF) of 20 after 10 d of exposure. During embryonic development, the eggshell acted as an effective shield against ²⁴¹Am penetration with 98% of the radionuclide associated with this membrane. In contrast, ¹³⁴Cs was transported across the eggshell but was not concentrated to any large extent in the eggs. The mean ¹³⁴Cs CF for the whole egg did not exceed 1.3 ± 0.2, indicating a poor affinity of ¹³⁴Cs for the embryonic tissues of cuttlefish.