The most abundant synapses in the central nervous system of vertebrates are inhibitory synapses that use the neurotransmitter gamma-aminobutyric acid (GABA). GABA is also an important neurotransmitter in C. elegans; however, in contrast to vertebrates where GABA acts at synapses of the central nervous system, in nematodes GABA acts primarily at neuromuscular synapses. Specifically, GABA acts to relax the body muscles during locomotion and foraging and to contract the enteric muscles during defecation. The importance of this neurotransmitter for basic motor functions of the worm has facilitated the genetic analysis of proteins required for GABA function. Genetic screens have identified the GABA biosynthetic enzyme, the vesicular transporter, inhibitory and excitatory receptors, and a transcription factor required for the differentiation of GABA cell identity. The plasma membrane transporter and other GABA receptors have been identified by molecular criteria.