Gamma-aminobutyric acid (GABA) is present in the central nervous system of Aplysia californica (Gastropoda, Opisthobranchia) where its role as a neurotransmitter is supported by pharmacological, biochemical, and anatomical investigations. In this study, the distribution of GABA-immunoreactive (GABAi) neurons and fiber systems in Aplysia was examined by using wholemount immunohistochemistry and nerve backfill methods. GABAi neurons were located in the buccal, cerebral, and pedal ganglia. Major commissural fiber systems were present in each of these ganglia, whereas more limited fiber systems were observed in the ganglionic connectives. Some of the interganglionic fibers were found to originate from two unpaired GABAi neurons, one in the buccal ganglion and one in the right pedal ganglion, each of which exhibited bilateral projections. No GABAi fibers were found in the nerves that innervate peripheral sensory, motor, or visceral organs. Although GABAi cells were not observed in the pleural or abdominal ganglia, these ganglia did receive limited projections of GABAi fibers originating from neurons in the pedal ganglia. The distribution of GABAi neurons suggests that this transmitter system may be primarily involved in coordinating certain bilateral central pattern generator (CPG) systems related to feeding and locomotion. In addition, the presence of specific interganglionic GABAi projections also suggests a role in the regulation or coordination of circuits that produce components of complex behaviors.