We shall compare the structural and functional differences and similarities in peptide transporters from bacteria to mammals. The E. coli peptide transporter (YgdR) is one of the 4 putative peptide transporters in the E. coli genome. When overexpressed, it shows essentially all features of the mammalian transporters with a PEPT1-like phenotype. We provide information on its oligomeric state and structure as derived by blue-native gel electrophorsis, by cross-linking studies and transmission electron microscopy. Carriers from various species (zebrafish, mouse, rabbit, human) when studied by electrophysiology in Xenopus oocytes show generally very similar features despite marked sequence differences. Since the physiological role of peptide transport as compared to transport of free amino acids in intestine and other tissues is still not well understood, we also analysed gene deletions of PEPT1 in C. elegans. Animals that lack the intestinal carrier show impaired growth and development despite an up-regulation of amino acid transporters (mainly for neutral amino acids) in the gut at the mRNA level and functional levels. When fed 13C-labelled bacteria, animals lacking intestinal peptide transport do display changes in amino acid metabolism as revealed by NMR and isotopologe analysis of free amino acids and those in body proteins. These differences may be caused by impaired signalling along the insulin-IGF axis in animals with a loss of intestinal peptide transport.