Plasma membrane proteins of the solute carrier family 5 (SLC5) are responsible for sodium-coupled uptake of ions, sugars and nutrients in the vertebrate body. Mutations in SLC5 genes are the cause of several inherited human disorders. We have recently reported the cloning and transport properties of SGLT-1L, a Xenopus homologue of the sodium-dependent glucose cotransporter 1 (SGLT-1) [Nagata et al. (1999) Am. J. Physiol. 276: G1251 -G 1259]. Here, we describe the phylogenetic relationship of SGLT-1L with other members of the SLC5 family and characterize its expression during Xenopus embryogenesis and in organ cultures. Sequence comparisons and phylogenetic analyses of all known vertebrate SLC5 sequences indicated that Xenopus SGLT-1L encodes a novel SLC5 member, which shares highest amino acid identity with mammalian ST-1 proteins. Temporal and spatial expression of SGLT-1L during Xenopus embryogenesis was examined by whole mount in situ hybridization. Initiation of SGLT-1L expression occurred in the late tailbud embryo. Remarkably, expression was restricted to the developing pronephric kidney. SGLT-1L was highly expressed in tubular epithelia, but completely absent from the epithelia of the duct. Analysis of growth factor-treated animal caps indicated that expression of SGLT-1L could also be induced in organ cultures. Taken together, our findings indicate that the expression of sodium-dependent solute cotransporter genes in early segments of the excretory system appears to be conserved between pronephric and metanephric kidneys. Furthermore, we establish SGLT-1L as a novel, highly specific molecular marker for pronephric tubule epithelia undergoing maturation and terminal differentiation in Xenopus.