For three plants from a peat bog (Trichophorum caespitosum, Molinia coerulea, Calluna vulgaris) the concentration of 137Cs, the ratio 137Cs:134Cs, and stable K was determined in intervals of about 14 d from June to November 1987. The results show that for two grasses, Trichophorum caespitosum and Molinia coerulea (which have only perennial roots but sprout every year while the old leaves wither), the concentration of 137Cs decreased considerably during the growing season (1800–240, respectively, 4000–320 Bq kg−1 dry weight). A remarkably similar behavior was observed for the seasonal variability of K and radiocesium in the two grass species, which resulted in a nearly constant ratio of 137Cs:K during the year. In contrast, for the evergreen plant Calluna vulgaris (heather) which was contaminated surficially by the Chernobyl fallout, the concentrations of K and 137Cs were rather constant during 1987 (leaves about 10,000; stems about 5000 Bq kg−1 dry weight), even though radiocesium was taken up by the leaves and transported within the plant. For the two grasses, the plant:soil concentration ratios (CR) were obtained separately for total 137Cs, 137Cs from the global fallout, and Chernobyl-derived 137Cs. The CR of 137Cs from the global fallout decreased for Trichophorum caespitosum from 1.9 in the spring to 0.08 in the autumn, and for Chernobyl-derived 137Cs from 1.4 to 0.2. For Molinia coerulea, a similar behavior was observed. Possible reasons for the seasonal variability of the CR values and the different behavior of 137Cs from the global fallout and from the Chernobyl debris are discussed. The CR predicted for these plants with help of the distribution coefficient Kd for the sorption of 137Cs by the peat soil (Baes equation) agreed fairly well with the observed average CR during the growing season.