A cDNA with sequence similarity to isocitrate lyase (ICL) genes was isolated from the unicellular eukaryotic green alga Chlamydomonas reinhardtii as a light-induced mRNA in the carotenoid biosynthetic mutant strain FN68. The 416 amino acid open reading frame shows significant sequence similarity to isocitrate lyases of bacteria (70%), molds (48%), yeasts (45%), and plants (47%). Expression of the Chlamydomonas ICL gene was tested in the mutant strain FN68, which when grown in the dark fails to accumulate carotenoids and is deficient in chlorophyll, and in CC400G, a strain that accumulates wild-type levels of carotenoids and chlorophyll. In vegetative CC400G cells, ICL mRNA accumulated to a high level in the dark and declined to a barely detectable level within 30 min of exposure to light. This response was more sensitive to white (tungsten filament) or red light than green or blue light, excluding cryptochrome and rhodopsin as the photoreceptor. These results are consistent with excitation by chlorophyll and/or a phytochrome-related photoreceptor. In vegetative FN68 cells, ICL mRNA abundance was very low in the dark, but increased dramatically in response to light. At intensities above threshold, excitation by far-red or red light-induced ICL mRNA accumulation to the highest levels. The threshold of the response was lowest for far-red and blue light. These results are consistent with excitation of a photochromic far-red-responsive pigment.