The chromosome breaking effect of mitomycin C, methyl methanesulfonate, maleic hydrazide, 8-ethoxycaffeine and gamma rays on the primary root meristematic cells of Nigella damascena was studied. All the agents tested except 8-ethoxycaffeine, produced relatively fewer aberrations, when compared to Vicia faba cells, though both the species have nearly similar total chromosomal length. Test for the presence of heterochromatin in Nigella gave negative results and it is interpreted that the observed differences between Vicia and Nigella are due to the presence and absence of heterochromatin in their chromosome complements respectively. The role of heterochromatin in the production of chromosome aberrations and its significance in evolution are briefly discussed.