The distribution of FMRFamide-like peptides was studied in the complete stomatogastric nervous system [the paired commissural ganglia, single oesophageal ganglion, and the single stomatogastric ganglion (STG)] of two decapod crustacean species, the spiny lobster Panulirus interruptus and the rock crab Cancer borealis, by using immunocytochemical techniques. Antiserum 231 from the O'Donohue laboratory and antiserum 671C (described here) gave essentially the same staining patterns. In the commissural ganglia of both species there were ten to 20 stained neurons and dense neuropilar staining. The oesophageal ganglion of the crab had four stained neurons. Lucifer Yellow backfills followed by immunostaining showed that the two larger stained neurons of the oesophageal ganglion sent processes into the inferior ventricular nerve. The two smaller neurons sent processes into the inferior oesophageal nerves. The oesophageal ganglion of the lobster had two stained neurons that sent processes into the inferior ventricular nerve as well. None of the somata of the STG stained in either species, but in both species stained fibers were seen in the stomatogastric nerve that entered the STGs and ramified profusely throughout the neuropil. In some preparations of the crab, a stained fiber was visible in the dorsal ventricular nerve. The amounts of the FMRFamide-like peptides found in all regions of the nervous system of P. interruptus and C. borealis were determined by radioimmune assay (RIA). Column chromatography and high-performance liquid chromatography suggest that, in both species, much if not all of the RIA-assayable material is accountable for by peptides that are larger and more hydrophobic than FMRFamide.