The common frog, Rana temporaria, spawning at altitudes up to 2600 m, is potentially threatened by UVB radiation. In laboratory experiments we found a specifically UVB absorbing substance (UVAS) in the skin of tadpoles with maximum absorption between 290 and 300 nm. The production of UVAS is induced by both visible light and ultraviolet radiation. The concentrations of UVAS found in tadpoles from the field are about as high as those found in tadpoles from laboratory experiments with UVB radiation under simulated mid-summer conditions. The existence of two powerful sun screen factors (UVAS and melanin pigmentation) may explain the high resistance of R. temporaria tadpoles to the intensive UVB radiation at high altitudes.