The modifications which accompany the growth ofNostoc punctiformein culture media containing various forms of mineral nitrogen were described by light and electron microscopy. The presence of 0.012M NaN03and 0.04M NH4Cl inhibit heterocyst formation, while lower doses reduce their frequency. Parallel to the disappearance of heterocysts is the disappearance of the aseriate stage, so that the whole culture is made by long filaments formed by vegetative cells and, in particular conditions, by cells similar necridia and discs. When grown in presence of nitrogen compounds, the vegetative cells contain numerous and large cyanophycin granules. The higher large amounts of nitrates induce the formation of particular vegetative cells deprived of cyanophycin and enveloped in abundant mucilage. Some of these particular cells are similar to analogous found in the gonidial zone of coralloid roots of Cycads. The absence of true spores in the different culture media is discussed.