Two of the processes responsible for the harmful effects of leaf litter on the aquatic environment were studied, namely (i) deoxygenation of the water by the polyphenoloxidase-polyphenol system present in leaves, followed by microbiological deoxygenation (such pollution, due to organic matter, being common), and (ii) direct toxicity for fish, probably of leaf compounds such as terpenes, saponosides or phenols. These effects were compared for dried foliage from two poplars ( Populus balsamifera and P. tremuloïdes) and from conifers harvested in summer. The deciduous leaves had high overall deoxygenating effects, oxidase activity and phenol content. The coniferous needles produced little deoxygenation except for larch needles, which were strong microbiological deoxygenators, but had low oxidase activity. The toxicity for fish ( Catostomus catostomus, C. commersoni) of leaf extracts from four species was tested. Lodgepole pine ( Pinus contorta), the least deoxygenating species, was the most toxic for fish. The Douglas-fir foliage was twice as toxic as P. tremuloïdes foliage, and white spruce ( Picea glauca) needles were the least toxic species of all the foliage types tested.