Abstract Researchers from the United States are evaluating the nonrubber-producing extract (resin) of the guayule bush (Parthenium argentatum Gray) as a potential wood protectant against marine and terrestrial wood-destroying organisms in Panama. Wood treated with guayule resin was not attacked, or only lightly attacked, by limnorians (crustacean borers) after 45 months in Limon Bay. After 19 months in the Bay of Panama, treated wood was undamaged by teredinids but heavily damaged by pholads; both are molluscan borers. Treated wood was not attacked by termites of the genus Coptotermes during 33 months of exposure in the Panamanian rain forest. Termites of the genus Heterotermes had not attacked wood treated with guayule resin after 45 months in the Panamanian rain forest. A 20-week laboratory evaluation of guayule resin for efficacy against brown-rot (Gleophyllum trabeum, Antrodia carbonica, Formitopsis cajanderi, Lentinus ponderosa) and white-rot (Dichomitus squalens, Trametes versicolor, Ganodermas sp) fungi was performed. Statistical analysis revealed a significant inhibition of decay of the treated wood by these organisms. A 12-week laboratory evaluation of guayule resin for efficacy against soft-rot fungi was also conducted. None of the treated wood was damaged by soft-rot fungi.