The in vivo emission spectra of sixteen species of Jamaican fireflies and four species of American fireflies have been measured with a photoelectric recording spectrometer. A simple technique of stimulation with ethyl acetate vapor was found to elicit bright continuous emission over a period of several minutes. Although the luciferin-luciferase cross-reactions were positive in all cases tested, peak intensity wavelengths show distinct species differences, ranging from 5520 to 5820 A. Widely separated emission peaks arise from the thoracic and abdominal organs respectively in the same animal, the click-beetle, Pyrophorus.