Effect of ethanol on calf thymus DNA (CT-DNA) has been studied in the probe length scale range 3.2 less than or equal to qR(g) less than or equal to 6.3 in sodium chloride/sodium citrate buffer (10 mM) adjusted to pH = 7.0 with the CT-DNA concentration fixed at 2 x 10(-8) M at room temperature (20 degrees C). The measured hydrodynamic radius R-h and radius of gyration R-g clearly indicate condensation behavior, and the plausible conformation has been inferred from the R-h/R-g ratio. A sharp conformational transition was observed at an ethanol/water concentration 40:60. The polydispersity index deduced from the width of particle size distribution shows a minima corresponding exactly to this concentration. The results are supported by UV absorption studies done at lambda = 260 nm.