Abstract Precise recording of polyphasic optical melting curves was carried out for three kinds of bacteriophage lambda DNA differing in length ( λ cI857 S7 , λ cIb2 and λ cIb2 b5 ). Each of denaturation steps in melting profiles was characterized by two parameters, the melting temperature and the relative size. Any difference in fine structures in melting profiles was not recognized between the intact λ cI857 S7 DNA and the DNA fragmented into halves. The change in fine structures in melting profiles caused by the deletions of the b2 and b5 regions agreed qualitatively well with the prediction based on the physical and the genetical maps of phage lambda chromosome. The combined results indicate that, first, the well-known linear relationship between melting temperature and G+C content may apply also to each of denaturation steps in polyphasic melting curves due to heterogeneity of nucleotide distribution in a single DNA species, and, second, the effect of molecular ends on melting fine structures can be neglected at moderate salt concentration ( 0.01 M ⩽ Na + ⩽ 0.2 M ) for such a high molecular weight DNA. The heterogeneous distribution of nucleotides was derived for λDNA and for its b2 and b5 regions.