In the cyanobacteriumAnabaena torulosa, sporulation occurred even during the logarithmic growth phase. Sporulation was initiated by differentiation of the vegetative cell on one side, adjoining the heterocyst followed by differentiation of the vegetative cell on the other side. Subsequently, spores were differentiated alternately on either side to form spore strings. The sequence of sporulation supports the previous notion that a gradient of spore maturation exists in cyanobacteria and also indicates that the gradient is manifested unequally on either side of heterocysts. Sporulation was absent or negligible in a minerally enriched medium but ocurred readily in a minimal medium. The extent of sporulation was inversely related to phosphate concentration. Sporulation was enhanced at higher temperature. Incandescent light, but not fluorescent light, greatly stimulated sporulation suggesting possible involvement of red light in spore differentiation. Addition of filtrate, from 5 to 8 day old cultures, to freshly inoculatedA. torulosa greatly enhanced sporulation indicating the influence of extracellular products in spore formation.