EN
The author tried to explain the mode of food uptake in Ergasilus sieboldi Nordmann using Fe⁵⁹ izotope incorporated in the tench red cells. The first phase of the experiments involved the determination of the izotope dosis, administered intraperitoneally, and the time required for its incorporation into the erythrocytes; in the second phase, the izotope was given to the infected fishes and detected by means of contact autography. Five films out of 11 showed a reddening suggesting the presence of Fe⁵⁹ in the parasites. The experiment III aiming at the quantitative measurement, performed in autumn, was unsuccessful, as the fishes failed to incorporate the izotope. The results obtained imply the existence of a seasonal rythm in the metabolism of tench and substantiate the parasitism of Ergasilus sieboldi.