Abstract 1. 1. 5-Hydroxytryptamine (5-HT) has been identified by a combination of fluorescence histochemistry, thin-layer chromatography, and spectrofluorimetric assay in the tissues of Aspiculuris tetraptera. 2. 2. The distribution of 5-HT has been compared with the histochemical distribution of acetylcholinesterase. 3. 3. The distribution patterns observed are consistent with the hypotheses that there is an exogenous (neurogenic) as well as an endogenous (myogenic) component to the coordination of nematode activity, and that two neurotransmitters, one excitatory (by membrane depolarization) and the other inhibitory (by membrane hyperpolarization), operate in the nematode neuromuscular junction. 4. 4. The evidence suggests that 5-HT rather than γ-aminobutyric acid (GABA) is the inhibitory neurotransmitter.