AbstractWhen sectored colonies of a high‐yieldingClaviceps purpureastrain, producing ergotoxine alkaloids in submerged cultures, were investigated significant differences in surface morphology, mycelial differentiation, number of nuclei and in alkaloid production were established among sectors obtained after segregation. Biosynthetically most active sectors showed sclerotia‐like structure and a polynuclear state in cells, while low‐yielding sectors were composed mainly of undifferentiated hyphae with only few nuclei per cell. In cells of high productive sectors the content of DNA was about four times higher than in sectors of lower act