The majority of vascular flowering plants are able to form symbiotic associations with arbuscular mycorrhizal fungi. These symbioses, termed arbuscular mycorrhizas, are mutually beneficial, and the fungus delivers phosphate to the plant while receiving carbon. In these symbioses, phosphate uptake by the arbuscular mycorrhizal fungus is the first step in the process of phosphate transport to the plant. Previously, we cloned a phosphate transporter gene involved in this process. Here, we analyze the expression and regulation of a phosphate transporter gene (GiPT) in the extra-radical mycelium of the arbuscular mycorrhizal fungus Glomus intraradices during mycorrhizal association with carrot or Medicago truncatula roots. These analyses reveal that GiPT expression is regulated in response to phosphate concentrations in the environment surrounding the extra-radical hyphae and modulated by the overall phosphate status of the mycorrhiza. Phosphate concentrations, typical of those found in the soil solution, result in expression of GiPT. These data imply that G. intraradices can perceive phosphate levels in the external environment but also suggest the presence of an internal phosphate sensing mechanism.