There are significant differences in nuclear DNA amount between Eu-Sorghum species. The DNA variation is considerable. For example Sorghum durra has 63 per cent more DNA than Sorghum roxburghii. Increase in nuclear DNA is accompanied by increase in the mass of the nucleolus and of non-nucleolar material including the chromosomes. The ratio of DNA to non-DNA material is constant between species. The DNA concentration per unit volume at interphase in somatic nuclei and per unit of chromosome length is significantly lower in the wild species S. sudanense and S. virgatum than in the cultivated species S. caudatum, S. conspicuum, S. durra and S. roxburghii. It is suggested that the lower DNA concentration may reflect a lower proportion of heterochromatin relative to euchromatin in the wild as compared with the cultivated forms.