The melting transition of the magnesium salt of DNA has been systematically examined in the presence of various types of anions. The addition of ClO4- to a concentration of 3.0 N results in the biphasic optical transition, with the first phase exhibiting rapid reversibility and independence of the DNA concentration. This subtransition, which is interpreted as an intramolecular condensation to a collapsed form of DNA, is followed by a DNA concentration-dependent aggregation reaction. The aggregation can be reversed by increasing the ClO4- concentration to 6.0 N while elevating the temperature to post-transition levels. Alternatively, both the collapse and the aggregation can be prevented by melting in the presence of trichloroacetate, the most strongly chaotropic solvent for DNA which has been reported (K. Hamaguchi and E. P. Geiduschek (1962), J. Am. Chem. Soc. 84, 1329). The forces responsible for mediating both the collapse and the aggregation are superficially similar to those involved in maintaining duplex stability. The collapsed form, in particular, possibly possesses features in common with the condensed structures which can be produced in aqueous solution of certain polymers, such as polyethylene glycol (Lerman, L.S. (1971), Proc. Natl. Acad. Sci. U.S.A. 68, 1886).