[en] Mosses, characterized by slow growth, are able to efficiently accumulate different radionuclides from their environment to a much higher degree than other vegetation. Consequently, mosses are sensitive bioindicators of radioactive contamination for various ecosystems. On 11th March 2011 a catastrophic earthquake and subsequent tsunami set into motion a series of tragic events in Japan including the severe accident at Fukushima Dai-ichi nuclear power plant. Radioactivity released into the air became involved into atmospheric processes and the transport of matter in the biosphere, causing global contamination. Determination of radioactivity in mosses by high resolution gammaspectrometry showed the presence of radiocaesium, 137Cs and especially 134Cs, for a first time after Chernobyl accident. Systematic, long-term measurements of 137Cs activities in mosses for the period after Chernobyl are the base for fitting the measured values in moss to the theoretical curve (Eq.1). The ecological half-life of 137Cs in moss was found to be about 978 days [1,2]. Figure 1 shows increasing 137Cs activity concentration with maximum value 1720 ± 1 Bqkg^-1 in October 2011. In Table 1 are presented results of 134Cs activity concentration in mosses that ranged from 0.084 up to 0.904 Bqkg^-1. The 134Cs:137Cs activity ratio ranged from 0.0018 to 0.0576. At the end of 2012 the 134Cs activity could not be measured any more. The study was conducted in the Radiation Protection Unit of the Institute for Medical Research and Occupational Health in Zagreb, as a part of an extensive monitoring program of the Croatian environment.(author)