ELISA experiments revealed that an antiserum raised against an achetakinin-analog could specifically detect the recently isolated Culekinin Depolarizing Peptide (CDP)-II from the mosquito, Culex salinarius. The characterization indicated that two different epitopes in the C-terminal region of achetakinin I and CDP-II are recognized. One epitope is the -F-Y-region, the other is the -P-W-region. Among the peptides isolated from C. salinarius, the antiserum reacts only with CDP-II. Pre-absorption tests of the antiserum with CDP-II in immunohistological stainings abolished the reaction, while tests with pre-immune sera did not cause any immunopositive reactions. In the mosquito head ganglia, immunoreactive neurons were detected in the pars lateralis, the optic lobe and the suboesophageal ganglion. Although some immunopositive axons extended into the nervi corporis cardiacii II, no immunoreactivity was observed in the retrocerebral complex. In the thoracic ganglia, immunoreactive neurons were found in the pro-, meso- and metathoracic neuromeres. No immunoreactivity was found elsewhere. With this study we demonstrate that CDP-II, isolated from a whole body extract, is truly a neuropeptide, and the data suggest that its function is neuromodulating or neurotransmitting rather than neurohormonal.