High resolution thermal denaturation profiles are presented for the DNAs of bacteriophages lambda and T7. It is concluded that the temperature increment in data gathering and the method of calculating results meet the requirements for quantitative recording of the large amount of information found in the thermal transitions of both DNAs. The high resolution derivative denaturation profiles of these bacteriophage DNAs demonstrate that individual subtransitions (thermalites) of natural DNA are Gaussian in form and have narrow transition widths. Curve resolution performed on these profiles indicates that the mean thermalite width (2 sigma) is 0.33 degrees C and that this breadth is relatively invariant. Transition widths are not influenced by the position of thermalites in the profile or by cation concentration in the range from 5 to 30 mM Na+. However, the relative position of thermalites within a denaturation profile is a function of the solution ionic strength. The distribution of lengths of the DNA sequences which these thermalites represent is broad, with a number average length of 900 base pairs. Although we find an approximate similarity between the number of thermalites in the denaturation profile of T7 DNA and the number of looping regions in the electron microscopic partial denaturation map of Gomez and Lang ((1972), J. Mol. Biol. 70, 239-251) we conclude that free solution thermal denaturation experiments can be compared only superficially to the mapping results.