Escherichia coli chemotaxis is mediated by membrane receptor/histidine kinase signaling complexes. Fusing the cytoplasmic domain of the aspartate receptor, Tar, to a leucine zipper dimerization domain produces a hybrid, lzTar(C), that forms soluble complexes with CheA and CheW. The three-dimensional reconstruction of these complexes was different from that anticipated based solely on structures of the isolated components. We found that analogous complexes self-assembled with a monomeric cytoplasmic domain fragment of the serine receptor without the leucine zipper dimerization domain. These complexes have essentially the same size, composition, and architecture as those formed from lzTar(C). Thus, the organization of these receptor/signaling complexes is determined by conserved interactions between the constituent chemotaxis proteins and may represent the active form in vivo. To understand this structure in its cellular context, we propose a model involving parallel membrane segments in receptor-mediated CheA activation in vivo.