Sodium coupled-phosphate transporters (NaPi cotransporters) facilitate the transport of inorganic phosphate (Pi) across cell membranes. NaPi-II cotransporter (SLC34) isoforms: NaPi-IIa (SLC34A1), NaPi-IIb (SLC34A2) and NaPi-IIc (SLC34A3) are involved in regulated Pi transport across vertebrate epithelia. Numerous NaPi-II sequences from fish and mammals are available through public databases, while sequences from other major vertebrate groups i.e. amphibians and reptiles are sparse. Understanding the evolution of NaPi-II isoforms and their differential expression in species and organs is dependent on increasing current knowledge of sequences from these groups. The aim of this study is to amplify and sequence NaPi-II from amphibians and reptiles and subsequently use the obtained DNA sequences to infer gene trees reflecting the evolution of this essential protein. We have targeted selected species and are currently isolating RNA from kidney and small intestine and amplifying NaPi-II by RT- PCR. Amplification products are cloned, sequenced and aligned with available sequences from databases and phylogenetic relationships are inferred using Bayesian inference. Sequences from the sea urchin Strongylocentrotus are used as outgroup and the cephalochordate Branchiostoma and the various vertebrate groups form the ingroup. Our results show that both sea urchin and cephalochordate sequences fall outside the vertebrate based classification of NaPi-II isoforms. Furthermore, that both IIa and IIb isoforms are present within Amphibia as well as within the amniotes (reptiles, birds and mammals) and fish (Chondrichthyes and Teleostei). The IIc isoform seems unique to the mammalian lineage. Within the amphibians our preliminary data suggest that isoform expression is linked to the evolutionary lineage rather than correlated to a specific organ or habitat. The new NaPi-II sequence data from amphibians and reptiles will be presented, along with an overview of NaPi-II evolution.