Like other plant-pathogenic bacteria, Xanthomonas oryzae pv. oryzae, the causal agent of bacterial leaf blight of rice, has hrp genes that are indispensable for its virulence. The hrp genes are involved in the construction of the type III secretion (T3S) apparatus, through which dozens of virulence-related proteins, called effectors, are directly secreted into plant cells to suppress and disturb plant immune systems and/or induce plant susceptibility genes. The expression of hrp genes is strictly regulated and induced only in plants and in certain nutrient-poor media. Two proteins, HrpG and HrpX, are known as key regulators for hrp gene expression. Great efforts by many researchers have revealed unexpectedly that, besides HrpG and HrpX, many regulators are involved in this regulation, some of which also regulate the expression of virulence-related genes other than hrp. Moreover, it has been found that HrpG and HrpX regulate not only hrp genes and effector genes but also genes unrelated to the T3S system. These findings suggest that the expression of the hrp gene is orchestrally regulated with other virulence-related genes by a complicated, sophisticated regulatory network in X. oryzae pv. oryzae.