Two major lichen compounds (diffractaic and usnic acids), isolated from Usnea longissima Ach. were tested against 4th instar larvae and adults of the Colorado potato beetle, Leptinotarsa decemlineata Say for 24, 48, 72 and 96 h under laboratory conditions. Durations and mortalities were recorded at various concentrations (1.25, 2.5, 5, and 10 mg. ml(-1)). Results showed that secondary metabolites of U. longissima had a significant insecticidal potential against larvae and adults of L. decemlineata. Mortality rates after 96 h of treatment, with the highest concentration (10 mg. ml(-1)) of diffractaic and usnic acids, were 100 and 70% for adults and 100 and 80% for larvae, respectively. No mortality was observed in the control treatment. Bioassay tests with diffractaic and usnic acids revealed that the 96 h median lethal concentration (LC50) values were 1.783 and 4.048 mg. ml(-1) for adults and 1.509 and 2.759 mg. ml(-1,) for larvae of L. decemlineata, respectively. The present results suggest that the lichen secondary metabolites may have a potential action for control of L. decemlineata 4th instar larvae and adults.