Salmonella enterica serovar Typhimurium initiates infection of a host by inducing its own uptake into specialized M cells which reside within the epithelium overlaying Peyer's patches. Entry of Salmonella into intestinal epithelial cells is dependent upon invasion genes that are clustered together in Salmonella pathogenicity island 1 (SPI-1). Upon contact between serovar Typhimurium and epithelial cells targeted for bacterial internalization, bacterial proteins are injected into the host cell through a type III secretion system that leads to internalization of the bacteria. Previous work has established that the prgH, -I, -J, and -K and orgA genes reside in SPI-1, and the products of these genes are predicted to be components of the invasion secretion apparatus. We report that an error in the published orgA DNA sequence has been identified so that this region encodes two small genes rather than a single large open reading frame. These genes have been designated orgA and orgB. Additionally, an opening reading frame downstream of orgB, which we have designated orgC, has been identified and partially characterized. Previously published work has indicated that the prgH, -I, -J, and -K genes are transcribed from a promoter distinct from that used by the gene immediately downstream, orgA. Here, we present experiments indicating that orgA expression is driven by the prgH promoter. In addition, using reverse transcriptase PCR analysis, we have found that this polycistronic message extends downstream of prgH to include a total of 10 genes. To more fully characterize this invasion operon, we demonstrate that the prgH, prgI, prgJ, prgK, orgA, and orgB genes are each required for invasion and secretion, while orgC is not essential for the invasive phenotype.