SUMMARYIn streams, amphibious Hyphomycetes colonize needles ofPinus strobus, Pinus resinosa, Picea glauca, andAbies balsameaslowly and sparsely. Levels of colonization are greatly increased if the needles are treated with cold chloroform, methanol, hot ethanol, hot water or sliced in half lengthwise, before immersion in the stream. Dry heat at 70 C and 110 C applied to needles in air-tight and well ventilated conditions does not significantly facilitate subsequent colonization in the stream. The cuticular and epidermal layers of needles pretreated with hot ethanol and hot water show progressive deterioration during 36 da of immersion in the stream; this was not observed in untreated needles. We suggest that the physical barrier presented by the cuticle and epidermis plays an important role in retarding colonization of conifer needles by amphibious Hyphomycetes.