Phosphorylation of neurotransmitter receptors can modify their activity and regulate neuronal excitability. Cyclin-dependent kinase 5 (cdk5) is a proline-directed serine/threonine kinase involved not only in neuronal development, but also in synaptic function and plasticity. Here we demonstrate that group I metabotropic glutamate receptors (mGluRs), which modulate post-synaptic signaling by coupling to intracellular signal transduction pathways, are phosphorylated by cdk5. In vitro kinase assays reveal that cdk5 phosphorylates mGluR5 within the domain of the receptor that interacts with the scaffolding protein homer. Using a novel phosphospecific mGluR antibody, we show that the homer-binding domain of both mGluR1 and mGluR5 are phosphorylated in vivo, and that inhibition of cdk5 with siRNA decreases the amount of phosphorylated receptor. Furthermore, kinetic binding analysis, by surface plasmon resonance, indicates that phosphorylation of mGluR5 enhances its association with homer. Homer protein complexes in the post-synaptic density, and their disruption by an activity-dependent short homer 1a isoform, have been shown to regulate the trafficking and signaling of the mGluRs and impact many neuroadaptive processes. Phosphorylation of the mGluR homer-binding domain, in contrast to homer 1a induction, provides a novel mechanism for potentially regulating a subset of homer interactions.