The well-characterized erythrocyte glucose transporter is also expressed in brain, adipocytes, kidney, muscle, and certain transformed cells, but not in liver, intestine, or the islets of Langerhans. Using as probe a cDNA encoding the rat brain glucose transporter, we isolated from a rat liver cDNA library a clone encoding a protein 55% identical in sequence to the rat brain transporter, and with a superimpossible hydropathy plot. We expressed this protein in an E. coli mutant defective in glucose uptake; the protein was incorporated into the bacterial membrane and functioned as a glucose transporter. This new transporter is expressed in liver, intestine, kidney, and the islets of Langerhans; immunofluorescence analysis showed that it is present in the plasma membrane of the insulin-producing beta cells. Insulinoma cells express, inappropriately, the erythrocyte glucose transporter, and we suggest that this may be related to their inability to secrete insulin in response to elevations in glucose.