Light is essential for initiation and continued growth of coremia of Penicillium isariiforme. Production of normal unbranched coremia requires predominantly unidirectional light; diffuse light leads to the development of feathery or branched forms. Coremia are positively phototropic as long as growth continues. They elongate by apical growth, at a rate which remains roughly constant and is the same in continuous illumination and in diurnal alternation of 12 hr. light and 12 hr. darkness. Sporulation, which requires light, occurs c. 1 cm. behind the growing point of the coremium. Continuous illumination gives a uniformly sporulating coremium, whereas diurnal alternation of light and darkness may lead to alternate fertile and sterile zones. The individual hyphae of the coremium tip are themselves phototropic, and the phototropic and photomorphogenetic behaviour of the coremium is largely explicable in terms of the photosensitivity of these individual hyphae. The role of hyphal anastomosis in coremium growth is examined, and the coremia of P. isariiforme and P. claviforme Bain, are compared.