Summary: Benzothiadiazole (BTH) is a novel chemical activator of disease resistance in tobacco, wheat and other important agricultural plants. In this report, it is shown that BTH works by activating SAR in Arabidopsis thaliana. BTH‐treated plants were resistant to infection by turnip crinkle virus, Pseudomonas syringae pv ‘tomato’ DC3000 and Peronospora parasitica. Chemical treatment induced accumulation of mRNAs from the SAR‐associated genes, PR‐1, PR‐2 and PR‐5. BTH treatment induced both PR‐1 mRNA accumulation and resistance against P. parasitica in the ethylene response mutants, etr1 and ein2, and in the methyl jasmonate‐insensitive mutant, jar1, suggesting that BTH action is independent of these plant hormones. BTH treatment also induced both PR‐1 mRNA accumulation and P. parasitica resistance in transgenic Arabidopsis plants expressing the nahG gene, suggesting that BTH action does not require salicylic acid accumulation. However, because BTH‐treatment failed to induce either PR‐1 mRNA accumulation or P. parasitica resistance in the non‐inducible immunity mutant, nim1, it appears that BTH activates the SAR signal transduction pathway.