Technical Abstract: The Mi-1 gene of tomato confers resistance against three species of root-knot nematode in tomato (Lycopersicon esculentum). Transformation of tomato carrying the Mi-1 gene with a construct expressing NahG, which encodes salicylate hydroxylase, a bacterial enzyme that degrades salicylic acid (SA) to catechol, results in partial loss of resistance to root-knot nematodes. NahG expression also renders tomato roots sensitive to exogenous SA. This sensitivity is most likely due to the production of catechol from SA as we have found that catechol is toxic to tomato roots. Benzothiadiazole (BTH), a SA analog, restores nematode resistance in Mi-1 roots transformed with NahG, but does not confer resistance to susceptible tomato roots. The localized cell death produced by transient expression in Nicotiana benthamiana of Mi-DS4, a constitutively lethal chimera of Mi-1 with one of its homologs, was inhibited by co-expression with NahG. These results indicate that SA is an important component of the signaling that leads to nematode resistance and the associated hypersensitive response.