The swimming behavior of Halobacterium halobium is controlled by light which acts through retinal photoreceptor proteins. The sensing of near-ultraviolet (u.v.) was proposed to be mediated by the thermally metastable intermediate SR-I373 that is formed upon orange light absorption by sensory rhodopsin-I (SR-I). In order to test the validity of this proposal, we analyzed the photochromic behavior of the functional near-u.v. receptor in situ by use of an automated cell tracking system. The system was specifically designed for detection of swimming reversals in individual cells and calibrated with a straight-swimming mutant of H. halobium. Quantitative analysis of the response of the cells to near-u.v. revealed that orange background light increased the number of active near-u.v. receptor molecules. The intensity-dependence of this effect fitted into the kinetic scheme of a photochromic receptor pigment. The half-life of the functional near-u.v. receptor species was determined under continuous orange background light and found to be similar to that of the SR-I373 intermediate of sensory rhodopsin-I in intact cells. These results clearly support the assignment of the near-u.v. receptor to SR-I373. The kind of kinetic analysis described here, might be a useful tool in assigning spectroscopic data of pigments to photoreceptor function also in other organisms.