Firefly luciferin is a specialized metabolite restricted to fireflies (family Lampyridae) and other select families of beetles (order Coleoptera). Firefly luciferin undergoes luciferase-catalyzed oxidation to produce light, thereby enabling the luminous mating signals essential for reproductive success in most bioluminescent beetles. Although firefly luciferin and luciferase have become widely used biotechnological tools, questions remain regarding the physiology and biochemistry of firefly bioluminescence. Here we report sulfoluciferin to be an in vivo derivative of firefly luciferin in fireflies and report the cloning of luciferin sulfotransferase (LST) from the North American firefly Photinus pyralis. LST catalyzes the production of sulfoluciferin from firefly luciferin and the sulfo-donor PAPS. Sulfoluciferin is abundant in several surveyed firefly genera as well as in the bioluminescent elaterid beetle Pyrophorus luminosus at a low level. We propose that sulfoluciferin could serve as a luciferin storage molecule in fireflies and that LST may find use as a new tool to modulate existing biotechnological applications of the firefly bioluminescent system.