Signal transduction in bacterial chemotaxis is initiated by the binding of extracellular ligands to a specialized family of methyl-accepting chemoreceptor proteins. Chemoreceptors cluster at distinct regions of the cell and form stable ternary complexes with the histidine autokinase CheA and the adapter protein CheW. Here we report the direct visualization and spatial organization of chemoreceptor arrays in intact Escherichia coli cells by using cryo-electron tomography and biochemical techniques. In wild-type cells, ternary complexes are arranged as an extended lattice, which may or may not be ordered, with significant variations in the size and specific location among cells in the same population. In the absence of CheA and CheW, chemoreceptors do not form observable clusters and are diffusely localized to the cell pole. At disproportionately high receptor levels, membrane invaginations containing nonfunctional, axially interacting receptor assemblies are formed. However, functional chemoreceptor arrays can be reestablished by increasing cellular levels of CheA and CheW. Our results demonstrate that chemotaxis in E. coli requires the presence of chemoreceptor arrays and that the formation of these arrays requires the scaffolding interactions of the signaling molecules CheA and CheW.