Abstract: We report site-testing results obtained in the nighttime during the polar autumn and winter at Dome C. These results were collected during the first Concordia winterover by A. Agabi. They are based on seeing and isoplanatic angle monitoring, as well as in situ balloon measurements of the refractive index structure constant profiles C2n(h). Atmosphere is divided into two regions: (1) a 36 m high surface layer responsible for 87% of the turbulence, and (2) a very stable free atmosphere above, with a median seeing of 0.36“ +/- 0.19” at an elevation of h=30 m. The median seeing measured with a differential image motion monitor placed on top of an 8.5 m high tower is 1.3“ +/- 0.8”.