SUMMARYThe developmental cycle ofNostoc muscorum,a nitrogen‐fixing blue‐green alga, is controlled by the spectral quality of illumination. Red light with peak activity at 650 mμ induces development of filaments from a nonfilamentous (aseriate) stage of the life cycle. Red‐light photoinduction is reversed by simultaneous or subsequent exposure to light from a broad band in the green region of the spectrum. Photo‐reversibility of the red‐light induction, by green light, decays very slowly, remaining at an appreciable level for over 24 hr after the primary stimulus.Allophycocyanin is indicated to be the photoreceptor for red‐light induction. One or more phyco‐erythrins may operate as photoreceptors for reversal of induction.The dosage response and wavelength dependence of developmental photocontrol inNostoc muscorumA indicate that a nonphotosynthetic mechanism is involved in both developmental photoinduction and its p