Summary—Total mRNA extracted from the brain of a marine worm,Nereis diversicolor, werein vitrotranslated using 2 cell‐free systems: rabbit reticulocyte lysate and wheat germ extract. Among numerous products newly translated in both systems, only one class of 70 kDa polypeptides immunoprecipitated when we used a mixture of 3 well defined antibodies raised against known sequences of the human prepro‐CCK. At the cellular level, using immunocytochemistry techniques, strong and moderate immunoreactivities were seen in perikarya located in various ganglionic nuclei of the worm brain. Immunoreactive nerve fibres were visible in the neuropile but not in the infracerebral region, a neurohemal area. Immunoreactions also appeared on perikarya located in the anterior and medial groups of the ventral nerve cord. Furthermore, immunolabeled cells were observed in the midgut. Interestingly, several colocalizations of materials immunologically related to human prepro‐CCK and CCK/gastrin were observed in the brain and the ventral nerve cord both in perikarya and in nerve fibres. We propose that, inNereisa polypeptide (molecular mass 70 kDa) is the large precursor of molecules related to those of the CCK/gastrin peptide