Morphological changes in cultures of Basidiobolus ranarum Eidam have been studied in relation to light. Initial growth and formation of hyphal bodies were not influenced by light, but subsequent behaviour was affected. In dark, hyphal bodies became bicellular and incipient zygospore formation occurred. Production of aerial hyphae was delayed, and those which developed were sterile. In light, however, hyphal bodies tended to stay unicellular and induction of aerial hyphae (presumptive conidiophores) was greatly stimulated. Light was found to be a near-absolute requirement for the induction of differentiation at the tips of conidiophores, but differentiation itself could be completed without light. For all morphogenetic actions, light of short wavelengths ( Some observations were made on the development of individual conidiophores.