The formaldehyde fluorescence technique has been used to demonstrate amine-specific fluorescence in the nervous system of three nematode species, Prionchulus punclatus Cobb, Panagrellus redivivus Goodey, and Aphelenchus avenae Bastian. Examination of some of the physical and chemical properties of this fluorescence has shown it to be due principally to the primary catecholamine dopamine. Dopamine and dopa decarboxylase were also detected biochemically in A. avenae. Dopamine has now been proposed as a putative neurotransmitter in a number of nematode species and the role of this and other biogenic amines in nematodes is discussed. Of the two principal enzymes involved in the metabolism of monoamines, catechol-O-methyltransferase was detected in both A. avenae and P. redivivus but monoamine oxidase could not be detected in these or other nematode species.