A potato gene, erg-1, was isolated from a cDNA library enriched by suppression subtractive hybridisation for sequences up-regulated one hour after inoculation of cv. Stirling leaves with Erwinia carotovora ssp. atroseptica (Eca). Erg-1 (Eca response gene 1) exhibits strong similarity to phi-1, previously reported to be up-regulated by addition of phosphate to phosphate-starved tobacco cells. Furthermore, erg-1 is similar to families of related genes in tomato, Arabidopsis, and rice, but shows no homology to sequences from animals, fungi, or prokaryotes. Northern analyses showed erg-1 to be tightly regulated and rapidly induced by Eca, and by both compatible and incompatible races of Phytophthora infestans. In addition, erg-1 is up-regulated by treatment with Erwinia carotovora culture filtrate, ethylene and salicylic acid, showing that it is regulated by more than one defence-related signalling pathway.