Studies on nativeBeggiatoa demonstrated diel vertical migration into, and out of, sediments at the bottom of warm spring pools. Laboratory experiments withBeggiatoa in natural sediments suggested that high light was the cause of the downward movement. The nature of this presumed photomotion was clarified by microscopic observation of individual filaments of nativeBeggiatoa at light/dark boundaries where the light was varied in intensity and quality. Using “white light”, a negative photo-response was demonstrated, and a dose-response curve was constructed which indicates an increasing response to light over three orders of magnitude of intensity. A coarse action spectrum implicated a pigment with a peak in the blue region as the receptor. Pure culture studies showed the negative response to be a step-up phobic one. The light intensity increase necessary to invoke reversals was a smaller percentage of the initial intensity for higher initial intensities. The light intensity levels and gradient strengths necessary to evoke reversals in single filaments were consistent with the hypothesis that the step-up response accounts for the disappearance in the field. This response has adaptive significance since full sunlight was completely inhibitory toBeggiatoa growth, even when filaments were aggregated in tufts. Dilute suspensions were also inhibited by as little as 5000 lux (fluorescent lamps).