Spinosad is a natural insecticide, produced via fermentation culture of the actinomycete Saccharopolyspora spinosa, with potential use against a number of forest pests including spruce budworm (Choristoneura fumiferana [Clem]). Persistence of spinosad was determined in terrestrial fate experiments conducted within a semimature stand of black spruce (Picea mariana [Mill.]) and balsam fir (Abies balsamea [L]) in the Acadian forest region of New Brunswick, Canada. Results of experiments established under full coniferous canopy and in a canopy opening indicated that spinosad dissipated rapidly following hyperbolic kinetics in both litter and soils and was not susceptible to leaching. Time to 50% dissipation estimates for spinosyn A ranged from 2.0 to 12.4 days depending upon matrix and experimental conditions. Spinosyn D dissipated to levels below quantitation limits (0.02 microg/g of dry mass) within 7 days in all cases. Sporadic low-level detection of the demethylated metabolites suggested that parent compounds were degraded in situ.