The yeastYPR192wgene, which encodes a protein (Aqy1p) with strong homology to aquaporins (AQPs), was cloned from nineS. cerevisiaestrains. The osmotic water permeability coefficient (P f ) ofX. laevisoocytes expressing the gene cloned from the Σ1278b strain (AQY1-1) was 5.7 times higher than the P f of oocytes expressing the gene cloned from other strains (AQY1-2). Aqy1-1p, initially cloned without its C-terminus (Aqy1-1ΔCp), mediated an ~3 times higher water permeability than the full-length protein. This corresponds to a 3-fold higher protein density in the oocyte plasma membrane, as shown by freeze-fracture electron microscopy. P f measurements in yeast spheroplasts confirmed the presence of functional water channels in Σ1278b and a pharmacological study indicated that this strain contains at least a second functional aquaporin.