Deterioration of frescoes inside a hypogean room of the Domus aurea in Rome was mainly caused by the growth of an algal association which persisted on the frescoes even in darkness. Following the proposal to utilize a new illumination system, the present research was undertaken to ascertain the effect of an incandescent lamp on the whole association in culture. For comparison a fluorescent lamp was also employed, and two low light intensities were tested. Observations on the development and morphology of the various algal species and analysis of the photosynthetic pigment content indicate that the <i>Lyngbya</i> spp. dominant in the association appeared to be highly adaptable to both lamps and intensities. Carefully limited use of the new lamp was advisable.