IP3 (inositol 1,4,5-trisphosphate) receptors form tetrameric, IP3-gated Ca2+ channels in endoplasmic reticulum membranes, and are substrates for several kinases, including PKA (cAMP-dependent protein kinase). Activation of PKA has been reported to either enhance or inhibit type III IP3 receptor Ca2+-channel activity, but, as yet, the sites of phosphorylation remain unknown. Here, we reveal that PKA phosphorylates the type III IP3 receptor at Ser916, Ser934 and Ser1832, and that, intriguingly, each site is located close to a putative surface-exposed peptide loop. Furthermore, we demonstrate that Ser934 is considerably more susceptible to PKA-dependent phoshorylation than either Ser916 or Ser1832. These findings define the sites at which the type III IP3 receptor is phosphorylated by PKA, and provide the basis for exploring the functional consequences of this regulatory event.