Cell size variations in three heterocystous blue-green algae, viz. Nostoc muscorum (ISU strain), N. muscorum (UW strain) and N. calcicola (a local isolate) have been surveyed under conditions permissive and non-permissive of heterocyst differentiation. A monomodal distribution pattern was observed for the two strains of N. muscorum under conditions not requiring heterocyst differentiation, while elemental nitrogen grown cultures retained a group of relatively larger cells in addition to this mode. N. calcicola could not be similarly viewed because of the formation of hormogonia-like slender filaments causing heterogeneity. The heterocyst size showed two major peaks in each case, the larger heterocysts being almost twice as voluminous as the smaller ones. Attainment of a critical cell size, and the cell division which follows, may be crucial for making a cell differentiate into a heterocyst.