Many gram-negative plant pathogenic bacteria employ type III secretion systems to deliver effector proteins directly into the host cell during infection. On susceptible hosts, type III effectors aid pathogen growth by manipulating host defense pathways. On resistant hosts, some effectors can activate specific host disease resistance (R) genes, leading to generation of rapid and effective immune responses. The biochemical basis of these processes is poorly understood. The HopX (AvrPphE) family is a widespread type III effector among phytopathogenic bacteria. We determined that HopX family members are modular proteins composed of a conserved putative cysteine-based catalytic triad and a conserved potential target/cofactor interaction domain. HopX is soluble in host cells. Putative catalytic triad residues are required for avirulence activity on resistant bean hosts and for the generation of a cell-death response in specific Arabidopsis genotypes. The putative target/cofactor interaction domain is also required for these activities. Our data suggest that specific interaction with and modification of a cytosolic host target drives HopX recognition in resistant hosts and may contribute to virulence in susceptible hosts. Surprisingly, the Legionella pneumophila genome was found to contain a protein with similarity to HopX in sequence and domain arrangement, suggesting that these proteins might also contribute to animal pathogenesis and could be delivered to plant and animal hosts by diverse secretion systems.