Neurotransmission in the brain is critically dependent on excitatory synaptic signaling mediated by AMPA-class ionotropic glutamate receptors (AMPARs). AMPARs are known to be associated with Transmembrane AMPA receptor Regulatory Proteins (TARPs). In vertebrates, at least four TARPs appear to have redundant roles as obligate chaperones for AMPARs, thus greatly complicating analysis of TARP participation in synaptic function. We have overcome this limitation by identifying and mutating the essential set of TARPs in C. elegans (STG-1 and STG-2). In TARP mutants, AMPAR-mediated currents and worm behaviors are selectively disrupted despite apparently normal surface expression and clustering of the receptors. Reconstitution experiments indicate that both STG-1 and STG-2 can functionally substitute for vertebrate TARPs to modify receptor function. Thus, we show that TARPs are obligate auxiliary subunits for AMPARs with a primary, evolutionarily conserved functional role in the modification of current kinetics.