It becomes clear that large DNA molecules exhibit discrete conformational change between the coil and globule states with the addition of a very small amount (with the order of 10(-5) M) of cationic surfactant, cetyltrimethylammonium bromide (CTAB). We use fluorescence microscopy as a tool of single molecular observation of double-stranded T4DNA in an aqueous environment. When the concentration of CTAB is less than 9.4 x 10(-6) M, all DNA molecules exhibit the extended coil state. Whereas, when the CTAB concentration is higher than 2.0 x 10(-5) M, only compacted DNA molecules in the globular state are observed. In the region between these two critical concentrations, the coil and globule states coexist in the solution. A small but apparent increase of the size of the DNA globule is noticed at the CTAB concentration higher than 10(-3) M, due to the penetration of CTAB molecules into the DNA globule. To study the dynamical aspect of coil-globule transition, the process of the structural change from the coil into the globule state is observed under the spatial gradient of the CTAB concentration. The formation of aggregates from two or more globules is noticed at high concentrations of surfactant above 1.6 x 10(-4) M. Below this concentration, the globules do not coalesce into an aggregate even if they collide with each other. The translational diffusion constant D of DNA molecules is measured from the time series of video frames of the fluorescence image. The hydrodynamic gyration radius xi(H) is evaluated from the D and the viscosity of the bulk aqueous solutions. The increase of the globule size at the higher CTAB concentrations above 10(-3) M is confirmed by the increase of xi(H) values.