Abstract: The concentration and distribution of ⁵⁴Mn in tilapia fish (Tilapiamossambica Poster)penaed prawn (Penaeus peniciltlatus Alcock) and Clams (Arcagranosa Linnus and Cyclina sinensis Gemlin) were studied, and the content of ⁵⁴Mnin some biochemical matters in the body of these organisms was measured. The resultsshowed that concentration coefficient of ⁵⁴Mn in the fish, prawn and the two clamswere in the same order of magnitude. After one month of excretion experiment, most of ⁵⁴Mn in the fish was completly excreted, however, in the prawn and the clamsonly half of ⁵⁴Mn content were excreted. The shells of two clams had a special abilityin absorption and concentration of ⁵⁴Mn. The degestive syrtems of all these organismshad highest concentroting and fastest excreting abilities, while their muscles had lowconcentration and slow excretion abilities. The content of ⁵⁴Mn was highest in thenuclei acid, but lowest in the organic acid.