Photoinduced deprotonation of the retinylidene Schiff base in the sensory rhodopsin I transducer (SRI-Htrl) complex results in formation of the phototaxis signaling state S373. Here we report identification of a residue, His166, critical to this process, as well as to reprotonation of the Schiff base during the recovery phase of the SRI photocycle. Each of the residue substitutions A, D, G, L, S, V, or Y at position 166 reduces the flash yield of S373, to values ranging from 2% of wild type for H166Y to 23% for H166V. The yield of S373 is restored to wild-type levels in Htrl-free H166L by alkaline deprotonation of Asp76, a Schiff base proton acceptor normally not ionized in the SRI-Htrl complex, showing that proton transfer from the Schiff base in H166L occurs when an acceptor is made available. The flash yield and rate of decay of S373 of the mutants are pH dependent, even when complexed with Htrl, which confers pH insensitivity to wild-type SRI, suggesting that partial disruption of the complex has occurred. The rates of S373 reprotonation at neutral pH are also prolonged in all H166X mutants, with half-times from 5 s to 160 s (wild type, 1 s). All mutations of His166 tested disrupt phototaxis signaling. No response (H166D, H166L), dramatically reduced responses (H166V), or inverted responses to orange light (H166A, H166G, H166S, and H166Y) or to both orange and near-UV light (H166Y) are observed. Our conclusions are that His166 1) plays a role in the pathways of proton transfer both to and from the Schiff base in the SRI-Htrl complex, either as a structurally important residue or possibly as a participant in proton transfers; 2) is involved in the modulation of SRI photoreaction kinetics by Htrl; and 3) is important in phototaxis signaling. Consistent with the involvement of the His imidazole moiety, the addition of 10 mM imidazole to membrane suspensions containing H166A receptors accelerates S373 decay 10-fold at neutral pH, and a negligible effect is seen on wild-type SRI.