Molecular dynamics simulations were performed, in the physiological condition (300 K and 1 atm), on nanotube segments of various diameters submerged in water. The results show that water molecules can exist inside the nanotube segments, and, most importantly, the water molecules inside the tubes tend to organize themselves into a highly hydrogen-bonded network, i.e., solid-like wrapped-around ice sheets. The disorder-to-order transition of these ice-sheets can be achieved purely by tuning the size of the tubes. The results also suggest that the nanotubes have the potential to be used as proton-conducting pores for a variety of biological applications. (C) 2002 Elsevier Science B.V. All rights reserved.