The chemotactic sensory system of Escherichia coli comprises membrane-embedded chemoreceptors and six soluble chemotaxis (Che) proteins. These components form signaling complexes that mediate sensory excitation and adaptation. Previous determinations of cellular content of individual components provided differing and apparently conflicting values. We used quantitative immunoblotting to perform comprehensive determinations of cellular amounts of all components in two E. coli strains considered wild type for chemotaxis, grown in rich and minimal media. Cellular amounts varied up to 10-fold, but ratios between proteins varied no more than 30%. Thus, cellular stoichiometries were almost constant as amounts varied substantially. Calculations using those cellular stoichiometries and values for in vivo proportions of core components in complexes yielded an in vivo stoichiometry for core complexes of 3.4 receptor dimers and 1.6 CheW monomers for each CheA dimer and 2.4 CheY, 0.5 CheZ dimers, 0.08 CheB, and 0.05 CheR per complex. The values suggest a core unit of a trimer of chemoreceptor dimers, a dimer (or two monomers) of kinase CheA, and two CheW. These components may interact in extended arrays and, thus, stoichiometries could be nonintegral. In any case, cellular stoichiometries indicate that CheY could be bound to all signaling complexes and this binding would recruit essentially the entire cellular complement of unphosphorylated CheY, and also that phosphatase CheZ, methylesterase CheB, and methyltransferase CheR would be present at 1 per 2, per 14, and per 20 core complexes, respectively. These characteristic ratios will be important in quantitative treatments of chemotaxis, both experimental and theoretical.