Low-temperature UV-visible and circular dichroism (CD) spectroscopy were employed to study the photocycle of native (wild-type, WT) bacteriorhodopsin (bR) and its mutant derivative D96N. For both bR forms, irradiation at −80°C yields the M 412 photocycle intermediate ( λ max = 412 nm). The CD spectrum matches the UV-visible absorbance, including the (vibrational) fine structure. Conversion of the M form into subsequent photocycle intermediates and the re-formation of the bR parent state are accomplished on stepwise temperature elevation. For WT bR, the CD spectrum of the M intermediate appears to be comprised of the 412 nm band and a long-wavelength component (with a broad maximum around 560 nm). This interpretation is complicated by the fact that, at −80°C, and increasing with increasing temperature, the long-wavelength CD absorption becomes negative, whereas no qualitative change of the CD band around 410 nm is observed. This new intensity interferes with the CD band of M 412 in this wavelength region. Further warming of the sample leads to the formation of another intermediate with a positive CD spectrum around 550 nm which, under our experimental conditions (low temperature, high pH), is expected to be the intermediate N 550. The UV-visible absorption of N 550 has a similar wavelenght, but lower extinction coefficient, than that of bR 568, whereas its CD spectrum has not yet been reported. The same experimental apparatus was employed for the D96N mutant. The UV-visible changes with increasing temperature are governed by the retarded M decay, as reported previously (Butt et al., EMBO J., 8 (1989) 1657). The “temperature-resolved” CD spectra of this mutant also reflect the retarded conversion of the M form into the bR parent state through the N form. The nearly unaffected UV-visible and CD spectra in the UV range and the concomitant changes in the long-wavelength CD signals are interpreted as the observation of an M → M′ conversion, which has been proposed by Varo and Lanyi ( Biochemistry, 30 (1991) 5008). Sodium azide addition to D96N, which accelerates the photocycle kinetics, induces a shift of the absorption band to shorter wavelengths ( λ max = 490 nm) which resembles the behaviour of bR at high pH (pH > 9.0).