The visual system of the honeybee contains gamma-aminobutyric acid-(GABA)-like and 5-hydroxytryptamine-(5-HT)-like immunoreactive tangential fibers of similar morphology. These tangential fibers belong to two different cell types, as revealed by a double labelling procedure on the same section with GABA- and 5-HT-antisera, raised in different animals. The common projection areas of both fiber types suggests a possible interaction of serotonergic and GABAergic pathways in an insect visual system.