The chemical composition of autumn‐shed willow(Salix fragilis)leaves was examined over a 56‐day period of immersion in a stream, and compared with the chemical composition of periphyton. Much of the carbohydrates, phenolic compounds, and tannins were leached from the leaves within the first week. Nitrogen and chlorophyllaconcentrations increased as a percentage of ash‐free dry mass throughout the study period, whereas protein concentration remained relatively constant. In feeding preference studies the larvae of the trichopteranOlinga feredayipreferred 56‐day willow leaves to periphyton, 7‐day, and 28‐day‐incubated leaves. The most notable difference between 56‐day leaves and the other leaves was their lower phenolic content. The preference for 56‐day leaves over periphyton may be related to the high ash content of the periphyton from this stream. We conclude that leaves of introduced willow trees can provide a preferred food source for New Zealand stream macroinvertebrates.