OZONE is phytotoxic: it is damaging to cell integrity and photosynthesis(1,2), causing leaf necrosis(3) and reducing crop yield(4). It has been implicated in forest decline(5), perhaps through interactions with stress ethene(6). Here we show that organic hydroperoxides (ROOH), which are products of ozone-alkene reactions(7-9), are present in the leaves of isoprene-emitting plants after exposure to ozone, but are not found in control plants grown in clean air. On the basis of earlier studies(6,7,10), we suggest that this reaction of ozone with biogenic alkenes to produce toxic ROOH could be one of the mechanisms by which damage to plants occurs. This could be particularly important in areas experiencing acidic deposition, where the stability of ROOH will be enhanced. This model may explain in part the die-back of tree species producing reactive alkenes, such as the red spruce, which emits isoprene(11-15) and monoterpenes(16), and the Norway spruce and silver fir, which are both prolific monoterpene emitters(17).