We report here the identification of a tachykinin-related peptide (TRP) in crustaceans. Direct MALDI-TOF MS with brain slices of the crayfish Procambarus clarkii indicated the presence of a peptide having an MS number of around 934. Quadrupole orthogonal acceleration time-of-flight (Q-TOF) MS/MS analysis implied the sequence to be APSGFLGMRamide, identical to that of CabTRP Ia, isolated previously from the crab Cancer borealis, and Pev-tachykinin, from the shrimp Penaeus vannamei. The peptide has been shown to be myoactive in the cockroach hindgut, but the structure of its precursor protein had not been elucidated. A cDNA encoding crayfish preproTRP was cloned, revealing that the 225-residue protein contains seven identical copies of the peptide APSGFLGMRamide. This is unique because TRPs identified in other invertebrates were known to exist in several related forms in each species. The conserved structure of TRP in crustaceans was confirmed by cloning preproTRP from the spiny lobster Panulirus interruptus. RT-PCR and Northern blotting analyses suggested that the crayfish preproTRP mRNA is expressed throughout the nervous system, and in situ hybridization studies of the brain revealed that the transcript predominantly localizes to cell clusters 11 (dorsal lateral cells) and 9 (ventral lateral cells).