The highly homologous neutral amino acid transporters KAAT1 and CAATCH1, cloned from the midgut epithelium of the Manduca sexta larva, are members of the Na⁺Cl^- dependent transporter family. Recent evidence indicates that transporters of this family form constitutive oligomers. CAATCH1 and KAAT1 give rise to specific kinds of current depending on the transported amino acid, the cotransported ion, the pH and the membrane voltage. Different substrates induce notably distinct transport-associated currents in the two proteins that represent useful tools in structural-functional studies. To determine whether KAAT1 and CAATCH1 form functional oligomers, we have constructed four concatameric proteins for electrophysiological analysis, consisting of one KAAT1 protein covalently linked to another KAAT1 (K-K concatamer) or to CAATCH1 (K-C concatamer) and vice-versa (C-C concatamer and C-K concatamer), and eight constructs where the two transporters were linked to YFP or CFP in N-terminus or C- terminus, in order to determine the oligomer formation and the relative distance between the different subunits by FRET analysis. Heterologous expression of the concatenated constructs, and coinjection of the original proteins in different proportions allowed us to compare the characteristics of the currents to that of the oocytes expressing only the wild type proteins. All the constructs were fully active and their electrophysiological behaviour was consistent with the activity as monomeric proteins. However the FRET studies indicate that these transporters form oligomers in agreement with the LeuT_Aa atomic structure, and confirm that the C-terminals of the adjacent subunits are closer than N-terminals.