Aquaporin-1 (AQP1) is a class of water channels within the aquaporin superfamily. These channel proteins have been found in a wide variety of tissues such as kidney, lung, eye secretory gland and intestinal epithelium as well as in vacuolar membranes of plants. The major function of these channel proteins is to transport water exclusively into and out of cells. Based on amino acid sequence, it has been predicted that aquaporins contain six lipid bilayer-spanning a-helices. Models for the molecular folding of AQP1 containing six and four transmembrane a-helices have been proposed previously. Our earlier projection map at 3.5Å resolution revealed eight high density peaks which we interpreted as the projections of seven transmembrane α-helices and an eighth possibly transmembrane segment. The juxtaposition of structures seen in the projection map prevented us from unambiguously determining the exact number of transmembrane helices based on the projection map alone. We report here the three-dimensional (3-D) map at ˜6 Å resolution as determined by electron crystallography.