Unicellular algae have evolved to express many forms of high-affinity phosphate transporters, and homologs of these proteins are broadly distributed in yeast, fungi, higher plants, and vertebrates. In this report, an effort has been made to characterize such a transporter gene, StPHO , in the marine diatom Skeletonema tropicum . The primers used for polymerase chain reaction were designed by referring to a homologous gene in a prasinophyte, and the full-length (1692 bp) cDNA of StPHO was then cloned and sequenced. Sequence alignments and secondary structure prediction indicated that StPHO is a gene that encodes a type III Na + /Pi cotransporter (SLC20 family). To study the function of StPHO , specific concentrations of inorganic phosphate (Pi) were used to alter the physiological status of S. tropicum . In each treatment, samples were collected for the measurements of StPHO mRNA, [PO 4 3− ], cell abundance, the maximal photochemical efficiency of photosystem II ( F v / F m ), and alkaline phosphatase activity (APA). The results indicated that the ambient [PO 4 3− ] strongly affected the population growth and related physiological parameters of S. tropicum . The transcription of StPHO was fully repressed when the [PO 4 3− ] was greater than 1 μM but increased approximately 100-fold when the ambient [PO 4 3− ] decreased to 0.02 μM. Within this [PO 4 3− ] range, the regression equations are Y = −0.6644 X + 0.9034 and Y = −0.5908 X + 0.8054 for Pi-starved and Pi-limited treatments, respectively. This trend of gene expression suggested that StPHO plays an important role in the uptake of [PO 4 3− ], and StPHO may serve as a useful molecular biomarker for Pi-stressed diatom populations in marine ecosystems.