Whole-mount immunohistochemical technique using antibody to serotonin (5-HT) had been used to map presumptive serotoninergic structures in the ventral abdominal and thoracic nerve cord of crayfish Procambarus clarkii. 5-HT immunoreactivity was detected in more than 30 cell bodies, numerous fibers and peripheral nerve endings of root plexus and neuropilar regions. Immunoreactive fibers are arranged in 3 pairs of rostrocaudal bundles. The median (MFB) and the lateral fiber bundles run longitudinally through the entire thoracic and abdominal nerve cord (first thoracic T1 to sixth abdominal A6 ganglia). The central (CFB) fiber bundles extend only from the subesophageal to the fourth thoracic ganglia. In the 4 anterior thoracic ganglia (T1-T4), two lateroposterior cell bodies send their major processes in the ipsilateral MFB. In the fifth thoracic (T5) and first abdominal (A1) ganglia, the pattern of reactive structures is similar. Two large anterior cells which send a single prominent process to join the ipsilateral MFB and 4 smaller posterior cells. In other abdominal ventral ganglia, immunoreactive structures are smaller and less labeled. Cell bodies are displayed in two kinds of arrangement giving the appearance of two distinct homogeneous groups of ganglia: an anterior group (A2-A3-A4) that contained two pairs of small neurons and a posterior group (A5-A6) that contained only a large unpaired medial neuron. These results were discussed in relation to the serotonin-like immunoreactivity pattern previously described in lobster by Beltz and Krawitz.