Sr⁹⁰ Content in sea water and Plankton and Concentration Factors in the Adriatic and Ligurian Seas During the Period from 1960 to 1964. - During the period from 1960 to 1964 the Sr⁹⁰ content in the water and in the plankton of the Adriatic and Ligurian-Tyrrhenian Seas was determined and the concentration factors were calculated. The same values were determined in the cuttle-fish bone, for comparison, being the cuttle-fish equally present in both seas, whereas the plankton composition varies remarkably in the same. An increase in the radioactivity due to Sr⁹⁰ in both Ligurian-Tyrrhenian and Adriatic Seas was noted beginning from the middle of 1961 and this increase reached its maximum value in 1963. The Adriatic water always shows a higher Sr⁹⁰ content as compared to the Ligurian-Tyrrhenian Sea, whereas the contrary happens with the plankton. This is true for both the absolute values and the concentration factor which in the Ligurian-Tyrrhenian Sea is 12 times higher than in the Adriatic Sea. The authors believe that this difference is due to the constant presence of Acantharia in the Tyrrhenian Sea. These protozoa must therefore be considered as specific scavangers of the strontium and its radioisotopes.