Since seasonal variation in the nitrogenase activity of Stereocaulon paschale is known, the infrastructure of Stigonema in the cephalodia was examined in thalli of S. paschale collected from under snow in late winter, and from bare ground in late autumn. In contrast to the Stigonema cells of the autumn material, those of the winter material were relatively devoid of osmiophilic (lipid) droplets and polyglucoside and cyanophycin granules, reflecting that carbon and nitrogen storage products had been metabolized. In young cephalodia dividing cells were common, while in old cephalodia degenerating cells predominated. Haustoria appeared in material collected in both seasons, but may be limited to degenerating Stigonema cells or to Stigonema cells with a low metabolic activity.