The scanning tunneling microscope can image uncoated DNA submerged in water. The grooves of the double helix were clearly resolved in images of the 146-base pair fragment extracted from calf thymus nucleosome. In contrast to images obtained with dry DNA, the helix pitch varied only a small amount (36 +/- 5 angstroms). The path of the helix shows considerable variation. It is quite straight when the molecules are densely packed, but it curves and bends in isolated molecules.