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 superimposible 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 p cells. lnsulinoma 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.