Direct observation of the mechanism and dynamics of photo-initiated DNA compaction and re-expansion using a light-responsive cationic surfactant has been achieved with fluorescence microscopy. The surfactant undergoes a reversible photoisomerization upon exposure to visible (trans isomer, relatively hydrophobic) or UV (cis isomer, relatively hydrophilic) light. Thus, surfactant binding to DNA and the DNA condensation that result can both be initiated and controlled with light illumination. The inherent kinetics of DNA conformational changes, directly visualized following the in situ light "trigger" of surfactant photoisomerization, are found to occur at rates of approximately 9 microm/s or 240 kbp/s, at or near rates that can be achieved in natural processes. Furthermore, observation of photo-initiated DNA compaction, free of the effects of shear or mixing, provides evidence of a condensation mechanism that nucleates at the ends of the macromolecule. Ethidium bromide displacement studies, employed to gain insight on the mode of interaction between the photo-surfactant and DNA, also reveal the importance of both electrostatic and hydrophobic forces in surfactant binding and DNA condensation.