The spectrum of the carotenoid shift generated under continuous illumination in the GIC mutant of Rhodopseudomonas sphaeroides, which has a single carotenoid, has been examined under a variety of conditions expected to alter the size of the membrane potential. If the difference spectrum observed was due to a species with the spectrum of the bulk pigment, it would correspond to a change of a variable proportion of the pigment to a form absorbing at a higher wavelength. The maximal change induced by light could be described as a shift of about 10% of the pigment by 7 nm to the red, assuming that the shifted species was spectrally identical to the bulk carotenoid. It is concluded that the changes seen are not easily compatible with a progressive red shift in the whole spectrum with increasing applied potential as would be expected from a simple linear electrochromic mechanism; alternative hypotheses are discussed.