The olfactory system plays important roles in various crustacean behaviors. Despite numerous studies on different aspects of the olfactory neural pathway, only the decapod-tachykinin-related peptide (decapod-TRP) has been identified as a neuromodulator in this processing to date. To establish the functions of other related neuropeptides, we initially performed cDNA cloning of FMRFamide-related peptide (FaRP) and allatostatin (AST)-like peptide from the crayfish Procambarus clarkii, followed by in situ hybridization (ISH) analysis of these peptides, along with decapod-TRP, orcokinin, and crustacean-SIFamide. Cloned FaRP cDNA encodes seven copies of C-terminal RN(F/Y)LRFamide-containing peptide, whereas AST-like peptide cDNA comprises 29 copies of AST-like peptide (–YXFGLamide) and three additional putative peptides. ISH analysis of the brain revealed specific expression of crustacean-SIFamide mRNA in most projection neurons (cell cluster 10), and predominant localization of other mRNAs to interneurons. The data suggest that the crustacean-SIFamide neuropeptide is involved in output of the deutocerebrum to the protocerebrum. Double-fluorescence ISH data further disclose that, in cluster 9, orcokinin is coexpressed in decapod-TRP-specific interneurons, whereas AST-like peptide-containing cells do not overlap with orcokinin-expressing cells. On the other hand, FaRP-expressing cells overlap with both orcokinin- and AST-like peptide-specific cells. In cluster 11, where signals for AST-like peptide are absent, a number of interneurons express both decapod-TRP and orcokinin, emphasizing a close relationship between these two factors with regard to olfactory processing, and possibly tactile and/or visual sensory systems. These characteristic expression patterns of neuropeptides support their distinct involvement in the modulation of olfactory processing.