ABSTRACT The effects of four Diploptera punctata allatostatin peptides on the stomatogastric nervous system of the crab Cancer borealis were studied. All of the peptides had similar actions on the activity of neurons involved in rhythmic movements of the pyloric region of the stomach, decreasing the frequency of the pyloric rhythm in a dose-dependent manner. Diploptera allatostatin 3 (D-AST-3) was slightly more effective than the others. The absolute change in the frequency of the pyloric rhythm depended on the starting frequency, demonstrating that the effect of D-AST-3 depends on the preceding physiological state of the preparation. The largest decreases were observed when the starting frequency was slower than 0.8 Hz. Whole-mount immunocytochemistry with anti-Diploptera allatostatin 1 antibodies demonstrated the presence of allatostatin-like peptides in the paired commissural ganglia, the unpaired oesophageal ganglion and the stomatogastric ganglion, and in their connecting and motor nerves. Dense processes were labeled in the stomatogastric ganglion, 12–19 cell bodies and neuropil staining were found in each commissural ganglion, two cell bodies were stained in the oesophageal ganglion and two pairs of cell bodies, the gastropyloric receptor neurons, were stained in peripheral nerves.