In membrane preparations of Halobacterium, the hydrolysis of cGMP is accelerated by activators of G-proteins, namely GTP, GTP-γ-S, and fluoroaluminate. This suggests a type of G-protein which acts on a phosphodiesterase. Light stimuli which evoke behavioral responses in intact bacteria influence the rate of cGMP hydrolysis. Using an antiserum raised against a peptide identical with one of the sequences presumably involved in GTP binding of most G-proteins, a cross reactive protein with an apparent molecular mass of 59 kDa could be detected on immunoblots. The results support the idea that a G-protein may be part of the photosensory transduction chain of Halobacterium [(1987) Biochim. Biophys. Acta 923, 222-232].