An action spectrum for light-induced carotenoid biosynthesis in Neurospora crassa was determined in 4 to 20 nm steps from 260 to 800 nm. Four-day, dark-grown mycelial pads of N. crassa were exposed to varying amounts of monochromatic radiant energy and time. After a 48-hour incubation period at 6 C, carotenoid content was assayed spectrophotometrically in vivo. The action spectrum has maxima at 450 and 481 nm in the visible range and at 280 and 370 nm in the ultraviolet. A pigment synthesized by Neurospora whose absorption spectrum resembles the action spectrum is fl-carotene.A model for the regulation of carotenoid biosynthesis in N. crassa is proposed which describes a mechanism by which f8-carotene could act as a photoregulator. This carotenoid is suggested to be both photoreceptor for and regulator of carotenoid biosynthesis.