Predictions of the carbon-nutrient balance hypothesis were tested using a study of within-species phytochemical variation in the arroyo willow,Salix lasiolepis. The prediction that a balance between nutrients (total protein) and carbon-based secondary metabolites (total phenols) should exist was supported using water treatment and fertilizer experiments and wild willow clones. Leaf nitrogen content and net photosynthetic rates of plants potted in soil in which parental plants grew was low, indicating that wild plants exist under relatively low nutrient status-high carbon balance conditions. The hypothesis also correctly predicted positive relationships between shoot length and phenols in glasshouse plants, wild plants, and plants in the water treatment experiment and negative relationships between shoot length and phenols in the fertilizer treatment experiment. Total phenolic glycosides, fragilin, picein, salicortin, tremulacin, and tremuloidin all correlated positively with shoot length in glasshouse plants on a carbon-biased balance, and male willows had generally lower levels of phenolic glycosides than females. Salicortin and tremulacin showed the strongest positive relationships with shoot length.