  Single strand breaks are induced in DNA plasmids, pBR322 and pUC19, in
aqueous media exposed to strong fields generated using ultrashort laser pulses
(820 nm wavelength, 45 fs pulse duration, 1 kHz repetition rate) at intensities
of 1-12 TW cm$^{-2}$. The strong fields generate, {\it in situ}, electrons and
radicals that induce transformation of supercoiled DNA into relaxed DNA, the
extent of which is quantified. Introduction of electron and radical scavengers
inhibits DNA damage; results indicate that OH radicals are the primary (but not
sole) cause of DNA damage.
