Molecular water channels (aquaporins) allow living cells to adapt to osmotic variations by rapid and specific diffusion of water molecules. Aquaporins are present in animals, plants, algae, fungi and bacteria. Here we present an electron microscopic analysis of the most ancient water channel described so far: the aquaporin Z (AqpZ) of Escherichia coli. A recombinant AqpZ with a poly(histidine) tag at the N terminus has been constructed, overexpressed and purified to homogeneity. Solubilized with octylglucoside, the purified AqpZ remains associated as a homotetramer, and assembles into highly ordered two-dimensional tetragonal crystals with unit cell dimensions a = b = 95 A, gamma = 90 degrees when reconstituted by dialysis in the presence of lipids. Three-dimensional reconstruction of negatively stained lattices revealed the p42(1)2 packing arrangement that is also observed with the human erythrocyte water channel (AQP1). The 8 A projection map of the AqpZ tetramer in frozen hydrated samples is similar to that of AQP1, consistent with the high sequence homology between these proteins.