We investigated the effects of carboxylic acids on the oviposition behavior of the spruce budworm, a major defoliator of coniferous forests in North America. Carboxylic acids have been implicated as semiochemicals involved in lepidopteran host finding and oviposition, and they occur as free acids in the epicuticular wax of host (Picea and Abies spp.) foliage where spruce budworm lay eggs. In a dual-choice laboratory bioassay, several straight chain and cyclic monocarboxylic acids, and two dicarboxylic acids, significantly enhanced oviposition. Peak activity was associated with saturated acids having 8–12 carbons. Unsaturated oleic and linoleic acids were also preferred. The lowest effective dosage occurred at 7.8 nmol/cm² (1 mM solution). At higher dosages (≥780 nmol/cm²), C₉–C₁₀ acids became strongly deterrent and some shorter-chain and longer-chain acids became stimulating. Electroantennogram responses to C₆–C₁₆ acids indicated that behaviorally active acids are detected by olfaction. The most active acids (C₈–C₁₂ and oleic) have not been reported in the free fatty acid fraction of host cuticular waxes. However, long-chain C₁₄–C₂₈ acids are present as free acids, but they elicited significant oviposition responses only at doses that exceeded their levels in foliage waxes. Spruce budworm preference for carboxylic acids may represent a nonspecific response common to Lepidoptera, which may have evolved because of the ubiquitous occurrence of carboxylic acids in plants.