The effects of an introduced willow tree species(Salix fragilis)on the densities of macroinvertebrates were examined in two Central Otago, New Zealand, streams during 1991. Significantly lower invertebrate densities and biomass were observed in willow‐lined sections of the streams than in nearby open sections in summer, autumn, and winter. This result was observed in riffles and pools, for most dominant species and nearly all functional feeding groups. The effect was not associated with differences in the amount of fine paniculate organic matter (< 1 mm), stone surface organic layer biomass, or chlorophyllaconcentration, which were similar it open and willow‐shaded sites. Amounts of coarse paniculate organic matter (> 5 mm) were significantly higher in willow‐shaded riffles, but this did not result in increased abundance or biomass of shredders. Willow trees reduced incident stream illumination by as much as 80%, but did not appear to influence water chemistry between open and willow‐shaded sites. The decreased invertebrate densities probably result from a decrease in average substrate size and/or a lowering of food production through shading effects.