Arrestins (Arrs) are ubiquitous regulators of the most numerous family of signaling proteins, G protein-coupled receptors. Two models of the Arr-receptor interaction have been proposed: the binding of one Arr to an individual receptor or to two receptors in a dimer. To determine the binding stoichiometry in vivo, we used rod photoreceptors where rhodopsin (Rh) and Arr are expressed at comparably high levels and where Arr localization in the light is determined by its binding to activated Rh. Genetic manipulation of the expression of both proteins shows that the maximum amount of Arr that moves to the Rh-containing compartment exceeds 80%, but not 100%, of the molar amount of Rh present. In vitro experiments with purified proteins confirm that Arr "saturates" Rh at a 1:1 ratio. Thus, a single Rh molecule is necessary and sufficient to bind Arr. Remarkable structural conservation among receptors and Arrs strongly suggests that all Arr subtypes bind individual molecules of their cognate receptors.