The effect of the blue and near ultraviolet reversible photoreaction on conidial development in Botrytis cinerea was studied by observing microscopically selected conidiophores. Conidiophore development was divided into six stages : when the developing conidiophores from stage 2 (i.e. a mature conidiophore) to stage 5 (i.e. a conidiophore with conidium initial) were exposed to blue light for a short time, conidiation was suppressed ; the conidiophores already formed de-differentiated to ' sterile' conidiophores with sharply pointed tips. The suppression of conidial development by blue light could be reversed by subsequent exposure to near ultraviolet light, and conidia then developed normally. This mycochrome system functioned reciprocally within the range of identified conidiophore developmental stages and near ultraviolet light acted only at the same developmental stage as was inhibited by blue light.