Blue light stimulates the accumulation of beta-carotene (photocarotenogenesis) in the fungus Phycomyces blakesleeanus. To be effective, light must be given during a defined period of development, which immediately precedes the cessation of mycelial growth and the depletion of the glucose supply. The competence periods for photocarotenogenesis and photomorphogenesis in Phycomyces are the same when they are tested in the same mycelium. Photocarotenogenesis exhibits a two-step dependence on exposure, as if it resulted from the additon of two separate components with different thresholds and amplitudes. The low-exposure component produces a small beta-carotene accumulation, in comparison with that of dark-grown mycelia. The high-exposure component has a threshold of about 100 J· m(-2) blue light and produces a large beta-carotene accumulation, which is not saturated at 2·10(6) J·m(-2). Exposure-response curves were obtained at 12 wavelengths from 347 to 567 nm. The action spectra of the two components share general similarities with one another and with those of other Phycomyces photoresponses. The small, but significant differences in the action spectra of the two components imply that the respective photosystems are not identical. Light stimulates the carotene pathway in the carB mutants, which contain the colourless precursor phytoene, but not beta-carotene. Carotenogenesis is not photoinducible in carA mutants, independently of their carotene content. This and other observations on various car mutants indicate that light prevents the normal inhibition of the pathway by the carA and carS gene products. The chromophore(s) for photocarotenogenesis are presumably flavins, and not carotenes.