The fruticose lichen Stereocaulon vesuvianum, growing on the slopes of Mt. Vesuvius (south Italy), was used as a biomonitor of 134Cs, 137Cs, 103Ru and 106Ru derived from the April 26 1986 Chernobyl nuclear reactor accident. Samples were taken at five different quotes (370, 490, 580, 780 and 960 m a.s.l.) and four successive dates (October 1986, December 1986, October 1987 and May 1999). At the first sampling, the concentrations (as Bq kg(-1) dry weight) ranged between 460 and 1020 for 134Cs, 1330 and 2500 for 137Cs, 90 and 200 for 103Ru and 360 and 710 for 106Ru, values generally lower in respect to those measured in soil and higher plants. Of the total 137Cs measured only 14% was due to 1950s and 1960s nuclear weapons tests fallout. Highest average activities of all nuclides were observed at the quote of 960 m and significant correlation (0.7<r(2)<0.9) with altitude was found for 106Ru and 137Cs values. The time span between the first two samplings (38 days) was enough to highlight only the decay of 103Ru (T1/2=40 days). At sampling performed 1 year later (October 1987) only 134Cs, 137Cs, and 106Ru were observed. Thirteen years after the Chernobyl incident (May 1999), although a decrease was obvious with time, the 137Cs content in lichen thallus was still appreciable (270-570 Bq kg(-1) dw). Experimental results from the first three samplings yield effective residence time of 0.98+/-0.07, 5.9+/-0.3 and 1.6+/-0.1 years for 106Ru, 137Cs and 134Cs, respectively. The last sampling of May 1999 allowed to achieve a more precise determination of lifetime of 137Cs (6.1+/-0.4 years).