A pigment‐dispersing hormone (PDH) from eyestalks of the crayfishProcambarus clarkiiwas purified by gel filtration, cation‐exchange chromatography, partition chromatography, and reversed‐phase HPLC. Based on automated sequencing and by the identical chromatographic behavior of the native PDH and the synthetic amidated form of the deduced sequence, the primary structure ofProcambarusPDH has been established as: Asn‐Ser‐Glu‐Leu‐Ile‐Asn‐Ser‐Ile‐Leu‐Gly‐Leu‐Pro‐Lys‐Val‐Met‐Asn‐Glu‐Ala‐NH2. This peptide differs from β‐PDH of the fiddler crabUca pugilatorat a single position, Glu17in place of Asp17. Because of this substitution,ProcambarusPDH was 4 to 7‐fold less potent than β‐PDH in causing pigment dispersion in the erythrophores, leucophores, and melanophores ofUca. In contrast,ProcambarusPDH was 4‐fold more potent than β‐PDH in eliciting pigment dispersion in the erythrophores of Procambarus. These peptides displayed less marked differences in potency in triggering leucophore pigment dispersion and light‐adaptational distal eye pigment movement inProcambarus. These findings indicate that the structural requirements for PDH‐receptor interactions vary with the sp