Spectral distributions of ambient underwater sunlight in Oyster Bay, Falmouth Harbor, Jamaica, West Indies, have been measured during the diurnal rise in the stimulable bioluminescent intensity of the marine dinoflagellate, Pyrodinium bahamense. In addition to the changes in light intensity during this period, there are large changes in the relative spectral distributions, as functions of both time and depth in the Bay. The blue-to-red intensity ratios vary by factors of 10 to 100.